Why Identifying the Specific Sound Matters Before Doing Anything Else
Why sound character, timing, and location all matter
Mitsubishi air conditioners produce sounds across a wide spectrum — from the quiet hum of normal inverter operation to the sharp metallic grinding of a mechanical failure in progress. Each sound type originates from a specific component or operating condition. Describing a noise as simply loud or unusual is rarely enough to identify its source. The character of the sound, when it occurs, and where it comes from are consequently the three pieces of information that narrow the cause accurately.
Many Melbourne homeowners switch their system off immediately when any unusual noise appears. Sometimes that is the correct response — for a grinding or banging sound from the outdoor unit, switching off protects the compressor from damage that costs significantly more than the repair the noise is signalling. For a crackling sound from the indoor unit on startup, however, switching off is unnecessary because the sound is a normal thermal expansion response that requires no action at all.
How to describe your noise accurately before calling
Three observations help identify any air conditioning noise before a technician visit. First, note the character of the sound: is it grinding, rattling, hissing, clicking, crackling, banging, buzzing, or squealing? Second, note when it occurs: does it happen at startup only, during continuous operation, when changing modes, or when the system switches off? Third, and importantly, note where it comes from: the indoor unit, the outdoor unit, or the connecting pipework.
These three pieces of information allow us to identify the most likely cause before arriving. They also help determine whether the system should continue operating while waiting for a service visit or whether switching off immediately is the safer choice.
The most urgent rule to remember
Any grinding, loud banging, or burning smell alongside a noise from either unit warrants switching the system off immediately at the wall isolator. These combinations indicate active mechanical or electrical stress. Consequently, continuing to operate through them converts a manageable repair into a more expensive one in a short period. Every other noise type in this guide allows a more measured response — noting the sound and booking a service rather than reacting immediately.
Ten Mitsubishi Air Conditioner Noises Explained With Causes and Actions
Each sound below is described by its character, its timing, and its most likely source. Select the sound that most closely matches what you are hearing to read the full explanation and the appropriate response for your situation.
Why This Sound Occurs
The indoor unit casing, vane assembly, and internal components expand when heat is introduced and contract when they cool. This dimensional change consequently produces audible crackling or clicking sounds at the points where plastic components meet. The sounds occur most noticeably at startup — when the unit transitions from ambient temperature to operating temperature — and again at shutdown when it cools.
The sound is most pronounced during Melbourne's temperature extremes, when the difference between the ambient temperature and the operating temperature is greatest. A system that has been cold overnight therefore produces more thermal expansion noise on its first morning startup than one that has been operating continuously.
How to Confirm It Is Normal
- The crackling occurs only at startup and shutdown, not during continuous operation
- No fault code accompanies the sound on the display or remote
- The system operates normally in all modes without any performance reduction
- The sound has been present since installation and has not changed in character or intensity
Action Required
None. Thermal expansion crackling is a normal characteristic of all plastic-cased indoor units and is not a fault. If the crackling has recently become louder or more persistent, or if it continues during steady-state operation rather than only at startup and shutdown, a panel clip may have become loose and a service visit to reseat it is worthwhile.
Why This Sound Occurs
When a Mitsubishi system switches between cooling and heating modes, the refrigerant flow direction reverses through the circuit. This directional change produces a brief hissing or gurgling sound as the refrigerant transitions through the expansion valve and the flow direction changes. The sound consequently occurs only during the mode transition and then stops completely.
The same sound can occur briefly during defrost cycles in heating mode. Melbourne winters occasionally trigger the outdoor unit defrost cycle when the outdoor temperature drops low enough for frost to begin forming on the outdoor coil. The transition out of defrost mode therefore produces a hissing or gurgling sound from the refrigerant circuit as it returns to normal heating operation.
How to Confirm It Is Normal
- The sound occurs only during mode changes or at the beginning and end of a defrost cycle
- Duration is brief and the sound stops completely afterward
- No fault code is present and performance returns to normal after the transition
- The sound comes from the connecting pipework or the indoor unit, not from outside the building independently of mode changes
Action Required
None for a brief transitional hiss during mode changes. A persistent hissing or bubbling sound that continues throughout operation and does not correspond to a mode change is a different situation covered under Sound 5 below.
Common Causes of Indoor Unit Rattling
Rattling from the indoor unit during continuous operation typically has one of three causes. A panel clip that has become loose allows the front grille to vibrate against the casing at specific fan speeds. Debris such as a small insect, a piece of leaf matter, or a dislodged component strikes the fan blade during rotation. Fan scroll accumulation, however, is the most common cause and the most significant.
Dust and biological matter accumulating on the fan scroll interior surface and blade leading edges creates an uneven mass distribution. The resulting vibration consequently varies in intensity across different fan speed settings and may produce a rattle at one speed that disappears at another.
Diagnosing the Specific Cause
- Panel clip: press firmly along the grille edge while the system runs. If the rattle stops when a specific area is pressed, the clip at that location is loose and needs reseating
- Debris: switch the system off and inspect the visible fan area with a torch through the discharge grille. Any visible foreign object requires removal before restarting
- Fan scroll accumulation: the rattle is consistent at a specific fan speed, disappears at other speeds, and the system has not been professionally serviced recently
Action Required
Reseating a loose panel clip is a homeowner task. Debris removal requires switching off and is straightforward if the object is accessible. Fan scroll cleaning requires partial disassembly and is therefore a standard professional service task. Book a service visit if the rattle persists after checking the panel clips.
What Causes Outdoor Unit Vibration
The outdoor unit contains a compressor and a condenser fan motor, both of which produce vibration during normal operation. This vibration transmits to the mounting surface and can amplify through it if the isolation pads between the unit and its bracket have deteriorated. Aged or compressed rubber isolation pads consequently lose their damping effectiveness and allow the unit's operating vibration to resonate through the bracket and wall structure.
A loose panel screw on the outdoor unit casing is another common cause. The high-frequency vibration of the compressor and fan motor is enough to work screws loose over time, particularly on units that have been in service for many years. These loose screws therefore allow the casing panel to vibrate at specific compressor frequencies, producing a buzzing sound that varies as the compressor speed changes.
Distinguishing Normal Hum From Abnormal Vibration
- Normal: a consistent low-frequency hum during operation that has been present since installation
- Abnormal: a vibration that has recently increased in intensity compared to previous seasons
- Abnormal: a buzzing that varies noticeably with compressor speed changes
- Abnormal: vibration that transmits visibly through the wall or floor structure to which the bracket is mounted
Action Required
Check and tighten any visible loose screws on the outdoor unit casing. Isolation pad inspection and replacement is a service task. If the vibration has increased significantly over a single season, a technician visit to assess the mounting, isolation pads, and compressor operation is worthwhile before the noise develops further.
What This Sound May Indicate
A persistent hissing or bubbling sound from the connecting pipework between the indoor and outdoor units — particularly if it continues during steady-state operation rather than only during mode changes — may indicate a refrigerant leak or a restriction in the refrigerant circuit. Refrigerant escaping through a small leak produces a hissing sound at the leak point. Air or moisture entering a circuit with a significant leak consequently produces a bubbling sound as these contaminants move through the system.
This is one of the sounds that requires professional investigation rather than a wait-and-see approach. A refrigerant leak that produces an audible sound has typically reached a level where the charge has dropped enough to affect system performance. The performance reduction may not yet be obvious to the homeowner, but a professional pressure check will confirm the deviation from specification.
Distinguishing From Normal Refrigerant Sounds
- Normal: a brief hiss at startup, shutdown, or mode change that stops once the transition is complete
- Concerning: a continuous hiss from the pipework that persists through steady operation
- Concerning: a bubbling or gurgling sound from the pipes during cooling or heating operation
- Concerning: the sound has developed recently and was not present in previous seasons
Action Required
Book a refrigerant pressure check. Do not add refrigerant without locating and repairing the leak source first, as this produces only temporary relief while the leak continues. A licensed refrigerant handler is required by Australian law for all refrigerant circuit work.
What Grinding Indicates
Grinding from either the indoor or outdoor unit during operation indicates mechanical contact between surfaces that are not designed to contact each other. In the indoor unit, this most often means fan motor bearings that have deteriorated to the point where the rotating shaft contacts the bearing housing directly. In the outdoor unit, grinding can indicate the same fan motor bearing failure or, more seriously, compressor internal wear.
Grinding is not a sound to monitor or investigate while the system continues operating. Every operating cycle while a grinding sound is present converts bearing material into metal debris inside the unit. That debris consequently migrates through the motor assembly and accelerates the rate of wear exponentially. A grinding sound identified early and addressed promptly is a bearing replacement. The same sound left unaddressed may involve the entire motor assembly or — in the case of outdoor unit compressor grinding — the compressor itself.
What to Do Immediately
- Switch the system off at the wall isolator immediately on identifying a grinding sound
- Do not restart the system to confirm the sound is still present
- Note which unit the grinding came from, indoor or outdoor, and the operating mode at the time
- Call for a same-day or next-day diagnostic visit
What the Repair Involves
Fan motor bearing replacement on an indoor unit is a same-visit repair in most cases, as fan motor assemblies for current Mitsubishi residential models are commonly stocked. Outdoor unit fan motor replacement follows the same approach. Compressor grinding is a more significant diagnosis that may involve compressor replacement or system assessment depending on the system's age and overall condition.
What Causes an Outdoor Unit Banging Sound
Loud banging from the outdoor unit indicates either a fan blade striking an object in the fan path or a liquid slug in the refrigerant circuit reaching the compressor. Both causes require immediate shutdown. A fan blade strike from a leaf, small debris, or a partially dislodged baffle can damage the blade balance. Repeated strikes consequently damage the blade itself and the fan motor shaft. A liquid refrigerant slug reaching the compressor produces a distinctive banging sound and can cause internal compressor damage.
What to Do Immediately
- Switch the system off at the wall isolator immediately
- Walk outside and inspect the outdoor unit for any visible debris near or inside the fan guard
- Do not reach inside the fan guard or attempt to remove debris while power is connected
- If no visible debris is present, the cause is internal and requires professional diagnosis before any restart
- Call for a same-day visit and describe the character of the bang — whether it was single or repetitive and the operating mode at the time
What Causes Fan Squealing
A squealing sound from the indoor unit fan typically indicates early-stage bearing wear in the fan motor. Unlike grinding — which represents advanced bearing failure — squealing represents the earlier stage where the bearing surface is beginning to deteriorate but has not yet reached metal-on-metal contact. Addressing a squeal before it develops into a grind is consequently significantly less expensive, because the motor may still be serviceable rather than requiring full replacement.
Fan scroll accumulation can also produce a squealing sound when the imbalanced fan assembly develops a resonance at certain operating speeds. This cause typically produces a sound that appears at one speed and disappears at another. Bearing wear, however, produces a sound that is consistent across speeds and typically worsens as operating time accumulates.
Action Required
- Book a professional service visit promptly on identifying a persistent squeal
- Note whether the squeal is consistent across all fan speeds or appears only at specific settings
- Reduce the system's operating hours while waiting for the service visit to slow the progression of bearing wear
- Do not apply lubricant or spray to the fan motor through the discharge grille, as this risks contaminating the motor windings
Outcome at Service
A service visit with a squeal symptom may resolve with fan scroll cleaning if accumulation imbalance is the cause. If bearing wear is confirmed, however, motor replacement during the visit is the appropriate outcome. We carry fan motor assemblies for current Mitsubishi residential models as standard stock in our service vehicles.
Why This Combination Is Always Serious
A burning smell alongside any operational noise indicates an electrical fault is active in the system. The smell may come from overheating wiring insulation, a capacitor that has reached its failure point, a motor winding breaking down under electrical stress, or a PCB component failing. Each of these conditions consequently creates a fire risk if the system continues to operate while the fault is active.
The noise accompanying the smell may be entirely unrelated, or it may indicate the mechanical stress that is producing the electrical overload. In either case, however, the burning smell is the critical signal that overrides any other diagnostic consideration.
What to Do Immediately
- Switch the system off at the wall isolator immediately
- If the smell is strong or smoke is visible, switch off the dedicated circuit breaker for the air conditioning at the switchboard
- Do not restart the system under any circumstances
- Call for a same-day service visit and describe the smell as well as the sound
- If smoke is visible from either unit, call your local fire service
When Water Sounds Are Normal
A gentle dripping sound from inside the indoor unit during cooling operation is the sound of condensate water dripping from the coil surface into the collection tray. This is normal and expected during any cooling cycle where significant humidity is being extracted from the room air. The sound consequently increases on more humid Melbourne summer days because more condensate is produced per hour of operation.
A brief trickling or gurgling sound as the system switches off is the sound of remaining condensate in the tray flowing through the drain pipe. This occurs particularly after extended cooling operation and stops within a short period of shutdown as the drain empties.
When Water Sounds Signal a Problem
- A dripping sound accompanied by visible water on the floor or ceiling below the unit is never normal
- A continuous dripping sound during operation that has increased in volume compared to previous seasons suggests a drain system that is partially blocked
- A splashing sound rather than a dripping sound indicates a higher volume of water than the tray and drain can handle, pointing to a drain blockage or a flooding coil defrost event
Action Required
Gentle internal dripping during cooling operation requires no action. Any water visible outside the unit, on the floor, or on the ceiling below requires switching the system off immediately and booking a service visit. For the complete water leak guide covering every cause and response, see our Mitsubishi water leak guide.
Sounds by Location Indoor Unit Versus Outdoor Unit
Knowing whether a sound originates from the indoor unit, the outdoor unit, or the connecting pipework narrows the diagnostic field significantly before any inspection begins.
Sounds From the Indoor Unit
Sounds From the Outdoor Unit
Sounds That Indicate Normal Operation Versus Sounds That Indicate a Developing Fault
The most common source of noise-related concern calls we receive involves sounds that are entirely normal. Understanding this distinction prevents unnecessary alarm and helps homeowners focus their attention on the sounds that genuinely warrant action.
These Sounds Are Normal
No action required unless the character changes
Crackling or clicking at startup on cold mornings caused by thermal expansion of the plastic casing
Brief hissing during mode changes as refrigerant flow direction reverses through the expansion valve
Quiet consistent hum from the outdoor unit condenser fan during operation
Gentle dripping inside the indoor unit from condensate falling into the collection tray
Brief trickling as condensate drains from the tray when the system shuts down
Water dripping from the outdoor unit base during heating mode, which is normal condensate production
A click or whirr from the vane motor as it repositions at startup or mode change
These Sounds Indicate a Fault
Book a service or switch off depending on urgency
Grinding from either unit during steady operation, indicating bearing failure in progress
Loud single or repetitive banging from the outdoor unit, indicating fan strike or compressor impact
Persistent hissing from the pipework that continues during steady-state operation rather than only at mode changes
Any noise accompanied by a burning or electrical smell from either unit
Rattling that has developed recently and was not present in previous seasons
Squealing that increases in intensity or frequency over successive operating days
A crackling sound that continues during steady operation rather than only at startup
Heard a noise you cannot identify from this guide?
Our Melbourne Mitsubishi specialists diagnose noise faults in a single visit.
The Right Response to Every Noise Category
This guide identifies three response categories for Mitsubishi air conditioning noises. Normal sounds require no action. Monitor sounds warrant a prompt service booking. Urgent sounds require switching off immediately and calling the same day. Understanding which category applies consequently prevents both the unnecessary panic that normal sounds produce and the delay that urgent sounds cannot afford.
Switch off immediately and call today for these sounds
- Any grinding sound from the indoor or outdoor unit during operation
- Loud banging from the outdoor unit, whether single or repetitive
- Any noise combined with a burning or electrical smell from either unit
- The circuit breaker trips alongside any noise when the system attempts to start
Book a service visit promptly for these sounds
- Rattling from the indoor unit that persists after checking and reseating panel clips
- Squealing from the indoor unit fan at any speed setting
- Persistent hissing from the connecting pipework during steady-state operation
- Outdoor unit vibration that has increased in intensity compared to previous seasons
- Any new sound that was not present during the previous season of operation
What Melbourne Homeowners Ask Most About Mitsubishi Air Conditioner Noises
Direct answers to the noise questions we receive most often, covering both the sounds that turn out to be nothing and the ones that genuinely need attention.
Almost certainly not — this is thermal expansion
A loud clicking on morning startup is the most commonly reported noise concern we receive, and it is almost always thermal expansion of the plastic casing. Melbourne's morning temperatures in autumn and spring can be considerably lower than the system's operating temperature, consequently producing a larger thermal differential at startup and therefore a louder expansion response.
When to investigate further
The sound is normal if it occurs at startup only and is not accompanied by any fault code or performance change. If the clicking continues during steady operation rather than stopping after the initial startup, a loose panel clip is the most likely cause. Press gently along the grille edges while the system runs to identify which clip has worked loose.
Three developing conditions account for most cases
Increased outdoor unit noise compared to previous seasons most commonly reflects one of three developing conditions. Isolation pad deterioration allows more of the compressor and fan motor vibration to transmit to the mounting surface and wall structure, amplifying the perceived sound level without the unit itself being louder. Condenser fan motor bearing wear consequently increases the mechanical noise the fan produces at any given speed. Condenser coil contamination forces the compressor to work harder to achieve the same heat rejection, increasing compressor noise as it operates at higher load levels.
How to respond
All three causes are addressed during a professional service visit. If the increase has been gradual over one or two seasons, a scheduled service is the appropriate response. If the noise increased suddenly within a single week, however, switch the system off and call for a diagnostic visit rather than a routine service booking.
Yes — in two distinct ways
A severely blocked filter creates a significant airflow restriction that forces the indoor fan to work against increased resistance. This increased load can cause the fan motor to produce a different sound character than it does under normal conditions — typically a more laboured hum. It can also cause the fan blade to vibrate as it tries to draw air through a highly restricted mesh, consequently producing a rattling or fluttering sound.
The secondary noise from a blocked filter
A blocked filter can also lead to coil freezing. When that frozen coil begins to melt after the system switches off, the rapid condensate release consequently produces a sudden increase in dripping and splashing sounds from the drain system. Cleaning the filter and restarting with a clean filter eliminates both of these noise contributors if they are the primary cause.
Almost certainly not — this is a normal mode-change sound
A brief hissing sound that occurs specifically during a mode change from cooling to heating — or vice versa — is the sound of refrigerant reversing its flow direction through the expansion valve and reversing valve. This is a normal operational sound on every heat pump system and does not consequently indicate a leak.
How to distinguish a normal hiss from a leak
A refrigerant leak produces a persistent hissing sound that continues during steady-state operation in either mode — not a brief transitional sound that occurs only during mode changes. If the hissing you hear corresponds precisely to a mode change or defrost cycle transition and then stops, it is normal. If the hissing is present throughout a cooling or heating cycle regardless of mode changes, a professional pressure check is warranted.
The remote off command test
Switch the system off from the remote and listen to where the sound stops. If the noise stops at the same moment the system receives the remote off command, the source is the indoor unit fan or components. If the noise continues for a short period after the remote command before stopping, the outdoor unit compressor and fan are still spinning down and the source may consequently be the outdoor unit.
The fan-only mode test
Alternatively, switch the system to fan-only mode, which operates the indoor fan without engaging the refrigerant circuit or outdoor unit. If the noise is present in fan-only mode, the source is therefore the indoor unit. If the noise disappears in fan-only mode, the source is the outdoor unit refrigerant circuit or compressor, which only operates in cooling and heating modes.
Most Mitsubishi Noises Have a Clear Cause and a Simple Response
Ten distinct noise types cover the vast majority of sounds Melbourne homeowners hear from their Mitsubishi systems. Most are normal. A few warrant a service booking. Two or three require switching off immediately. Matching your noise to the right category is consequently the difference between an unnecessary callout and a missed opportunity to prevent a developing fault from becoming an expensive repair.
Our Melbourne team diagnoses noise faults in a single visit across all Mitsubishi models. Call 03 4232 6971 with a description of the sound, when it occurs, and which unit it comes from. A phone recording of the noise is therefore the most useful preparation for any noise diagnostic call. For related reading, see our water leak guide.