Refrigerant gurgling, expansion ticking and defrost noises are normal. Rattles, grinding, screeching and new vibration are not. Noise reaching a neighbour is a separate issue and it is best resolved early.
Normal noises
| Noise | Cause | When |
|---|---|---|
| Gurgling or trickling | Refrigerant flowing through the expansion device | During operation, especially at start and stop |
| Ticking or clicking | Plastic components expanding and contracting with temperature | At start-up and shutdown |
| A hiss when it stops | Refrigerant pressures equalising | Immediately after shutdown |
| A whoosh or thump from outdoors | Reversing valve changing over | When switching between heating and cooling, or entering defrost |
| Outdoor unit steaming, fan stopping briefly | Defrost cycle in heating mode | In cold damp weather |
| A steady low hum | Compressor running | Continuously during operation |
| Fan speed changing | Inverter modulating to match the load | Normal and desirable |
| Water dripping outdoors | Condensate and defrost water | In heating mode, in cold weather |
A correctly sized inverter system spends most of its time running slowly, which is quiet. A system that is frequently loud is often oversized and cycling, or working hard because the outdoor unit cannot reject heat. Persistent loudness is worth investigating rather than accepting.
Noises that mean something
- Rattling – loose panels, loose fixings, or debris in the outdoor unit. Usually simple and worth dealing with before vibration loosens something further.
- Grinding or rumbling from the outdoor unit – fan motor bearings, or in a worse case the compressor. Grinding is not a noise to live with.
- Screeching or squealing – a bearing, or a belt on the rare belt-driven unit. Investigate.
- A loud buzz that was not there before – an electrical component, a contactor, or a loose panel resonating. Electrical buzzing warrants attention.
- Clattering from the outdoor fan – a damaged or unbalanced blade, or something caught in it. Turn it off and look.
- A new vibration transmitted into the building – failed anti-vibration mounts, or a fixing that has worked loose. Structural transmission is much more intrusive than airborne noise and it is usually cheap to fix.
- Bubbling or excessive gurgling that is new – can indicate low refrigerant charge, which means a leak.
- Water noises indoors – the condensate drain is not draining freely.
The general rule: a noise that has always been there and has not changed is probably normal. A noise that is new, or that has got worse, is worth investigating.
Noise and neighbours
This is where air conditioning causes genuine trouble, and it is almost always about siting rather than about the equipment.
Why it becomes a problem:
- It is continuous. Unlike a lawnmower or a car, it runs for hours.
- It runs in the evening and at night, when background noise falls and everything is more audible.
- It is mechanical, and mechanical sound is more noticeable than natural sound at the same level.
- Hard surfaces reflect it. A unit in a narrow gap between two houses is amplified rather than dispersed.
- Structural transmission through a party wall is worse than airborne noise and harder to ignore.
- It is somebody else’s choice imposing on them, which affects how it is perceived.
Environmental health can act on noise as a statutory nuisance, regardless of whether the installation had planning permission or complies with permitted development noise conditions. A complaint can therefore be made about a compliant installation, and the test is whether it amounts to a nuisance rather than whether it met a condition at installation.
Almost every dispute we hear about could have been avoided by a conversation before installation, or by taking a complaint seriously the first time it was made. A neighbour who has been ignored escalates; one who has been listened to usually accepts a reasonable mitigation.
Fixing a noise complaint
In order of effectiveness:
- Relocate the outdoor unit. By far the most effective and the most expensive. Distance and line of sight both matter enormously.
- Enable night or quiet mode. Most modern units have one, reducing fan and compressor speed at the cost of some capacity. Free, and frequently sufficient.
- Fit or replace anti-vibration mounts. Essential where the noise is being transmitted structurally rather than through the air, and cheap.
- Fit an acoustic barrier between the unit and the receptor – a solid barrier that blocks line of sight without obstructing the unit’s airflow. Design matters: a barrier that restricts airflow makes the unit louder, not quieter.
- Service it. A fouled condenser makes the unit work harder and run louder. Sometimes the complaint is the symptom of a maintenance issue.
- Check the fixings and panels, because a rattle is far more irritating than a hum and it is usually free to fix.
- Schedule around the complaint where practical – avoiding night running if the issue is a bedroom.
- Acoustic enclosure, as a last resort, designed to maintain full airflow clearances.
The mitigations that involve restricting airflow are the ones to be careful about. An enclosure that suffocates the unit trades a noise problem for an efficiency and reliability problem, and often does not even solve the noise because the unit is now working harder.
Getting it right at installation
- Site it away from bedroom windows – yours and theirs.
- Avoid narrow passages and enclosed courtyards, where sound reflects rather than disperses.
- Avoid party walls for structural transmission reasons.
- Use anti-vibration mounts as standard, not as an upgrade.
- Consider the unit’s rated sound power, which manufacturers publish – quieter models exist and the difference is real.
- Ground mounting generally transmits less into the building than wall mounting.
- Talk to the neighbour if it will be anywhere near a boundary. It costs nothing.
See where the outdoor unit goes.
New noise from the outdoor unit? Turn it off and look before it becomes a bigger repair. Rattles are cheap; grinding is not.
Questions we get asked about this
Is gurgling normal?
Yes – it is refrigerant flowing through the expansion device, most audible at start and stop. A sudden increase in gurgling can indicate low charge, which means a leak.
Why does my outdoor unit steam in winter?
Defrost cycle. In heating mode the outdoor coil gets cold enough to frost, and the system periodically reverses briefly to melt it. The steam and the water are both normal.
My neighbour has complained. Do I have to do anything?
Environmental health can act on noise as a statutory nuisance regardless of whether the installation complied when fitted. More practically, a complaint taken seriously early is far cheaper to resolve than one that escalates.
Can I build a box around the unit to quieten it?
Only an enclosure specifically designed to maintain full airflow clearances. A box that restricts airflow makes the unit work harder and run louder, which is the opposite of what you wanted.
The unit vibrates through the wall into the house.
Structural transmission, usually from failed or missing anti-vibration mounts, or a fixing that has worked loose. It is cheap to fix and much more intrusive than airborne noise.
Does night mode work?
It reduces fan and compressor speed, which genuinely reduces noise at the cost of some capacity. It is free to enable and frequently resolves a complaint on its own.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.