The short answer
Soft airflow, light clicks as plastic warms and cools, and hisses or gurgles as refrigerant and water move are all normal, as is a whoosh during defrost. Rattling, grinding, squealing, scraping, banging or loud buzzing usually mean something is loose, worn or failing.
On this page
Every air conditioner makes some sound. A fan moves air, a compressor pumps refrigerant, and plastic and metal expand and contract as they change temperature. The useful questions are what kind of sound it is, when it happens and whether it's new.
Normal sounds
| Sound | When you hear it | What it is |
|---|---|---|
| Rush of air | While running | The fan, louder at higher speeds |
| Beep | Using the remote | The unit accepting a command |
| Soft whirr | Switching on or off | The louvres moving |
| Click or creak | Starting, stopping, changing mode | Plastic expanding or contracting |
| Hiss or trickle | Starting, stopping, defrosting | Refrigerant flowing or settling |
| Gurgle | Cooling or dry mode | Water in the drain, or refrigerant flow |
| Whoosh, then quiet | Heating on a cold day | The system switching into defrost |
| Hum that rises and falls | Outdoor unit running | The compressor changing speed |
| Short hum at intervals | Where a pump is fitted | The condensate pump emptying |
Manufacturers' sound pressure figures put quiet indoor units at around 19–26 dB(A) on their lowest fan speed, and outdoor units typically at 45–50 dB(A) at 1 metre, so some sound is expected. How loud air conditioning is puts those figures in context.
Some units keep the indoor fan running for a short while after you switch off. The outdoor fan can also stop and start by itself, during normal running and during a defrost.
Sounds that aren't normal
| Sound | Likely cause | What to do |
|---|---|---|
| Rattle | Loose panel, filter, fixing or debris | Check what you can see, then an engineer |
| Grinding or squealing | Worn fan motor or bearing | Switch off, call an engineer |
| Scraping | Fan catching ice, debris or casing | Switch off, call an engineer |
| Banging or knocking | Loose part or compressor fault | Switch off, call an engineer |
| Loud buzzing or crackling | Possible electrical fault | Switch off at the isolator, call an engineer |
| Constant hiss, poor cooling or heating | Possible refrigerant leak | Switch off, call an engineer |
| Vibration through the wall | Worn mounts or loose bracket | Call an engineer |
| Repeated clicking, won't start | Electrical or control fault | Call an engineer |
Checks you can make
- Fan speed and mode. A high fan speed or a boost mode is noticeably louder. Try a lower speed, or a quiet or night mode if the unit has one, as described in sleep and quiet modes.
- The front panel and filters. A panel that isn't fully closed, or a filter that isn't clipped in properly, can rattle or whistle.
- Around the outdoor unit. Plants, a fence panel, loose pipe covers or something stored against the casing can buzz or rattle as the unit runs.
- Debris in the outdoor grille. Switch the system off first, and only remove what you can reach without putting fingers or tools through the grille, because the fan can start without warning.
- Defrost. A whoosh and hiss from the outdoor unit on a cold day, with steam and the indoor fan pausing, is a defrost and needs nothing from you.
If a noise does need an engineer, note when it happens, which unit it comes from and what mode the system was in, and record a short video on your phone. A sound that comes and goes is hard to diagnose if it stops while the engineer is there.
Outdoor unit noise and your neighbours
In England, an outdoor unit fitted under permitted development has to meet the MCS 020(a) noise standard: no more than 37 dB(A) at 1 metre from the nearest window or door of a habitable room in a neighbouring home. The installer keeps the calculation, and you should have a copy. Scotland, Wales and Northern Ireland have their own rules.
The calculation starts from the manufacturer's sound power figure for the unit. A unit that has become louder over time, perhaps because of worn mounts or a failing fan, is making more noise than that figure allowed for, so have it checked. Noise rules and your neighbours explains the standard in more detail.
Related guides
- How loud is air conditioning?Quiet indoor units are often around 19–26 dB(A) on their lowest fan speed and louder at higher speeds, while outdoor units are typically 45–50 dB(A) at 1 metre. How loud it seems also depends on where the units are and how hard they're working.
- Noise rules and your neighboursIn England, a heat pump fitted without planning permission must be no louder than 37 dB(A) at 1 metre from a neighbour's nearest habitable-room window or door, worked out with the MCS 020(a) calculation. Councils can also investigate noise complaints separately.
- Looking after the outdoor unitKeep the space around it clear so air flows freely, brush off leaves and snow with the system switched off, and leave cleaning the coil and anything inside the casing to an engineer. Otherwise it needs little beyond its regular service.
- When to call an engineerCall an engineer when simple checks don't fix a problem, and switch off straight away for burning smells, smoke, a circuit breaker that keeps tripping or signs of a refrigerant leak. Refrigerant, electrics and anything inside the casing are work for a qualified engineer.