“Will it keep me awake?” and “Will the neighbours hear it?” are two of the questions we hear most at surveys. Both are fair. A good indoor unit on low fan can be barely noticeable, and a well-placed outdoor unit is easy to live with. Put the same equipment in the wrong place, though, and it becomes a nuisance.
This guide explains how noise figures work, what makes indoor and outdoor units louder or quieter, the rules on noise and neighbours, and which sounds mean you should call an engineer.
Reading the figures
Noise is measured in decibels, usually A-weighted, written dB(A), to match how our ears hear. The scale is logarithmic. A rise of 3 dB is a doubling of sound energy, and a rise of about 10 dB sounds roughly twice as loud to most people. So a few decibels on a data sheet make more difference than they seem to.
Manufacturers publish two kinds of figure. Sound pressure is what you’d hear at a stated distance. Sound power is the total noise the unit emits, and is always a bigger number. Compare like with like: sound pressure with sound pressure, at the same fan speed, for units of the same capacity. Always check the manufacturer’s own data sheet for the exact model.
What you hear from the indoor unit
For bedrooms, the key figure is the sound pressure at the lowest fan speed, often called quiet or silent mode. Most of the noise is air moving through the fan and past the louvres, so running on a lower fan speed makes a big difference.
Other sounds are normal: a soft click as the louvres move, a gentle trickle of refrigerant, a creak as plastic expands and contracts when the unit changes temperature. A condensate pump, where fitted, adds a short hum when it runs. A good installer places the pump where it won’t be heard from the bed.
What makes an outdoor unit louder
- Reflecting surfaces. A unit tucked into a corner or a narrow passage bounces sound off the walls around it.
- The wall it’s fixed to. Brackets on a thin or hollow wall can carry vibration into the building. Anti-vibration mounts help.
- Distance. Noise falls off as you move away, so a few extra metres from a window helps.
- Load. An inverter unit runs quietly at part load and louder when working hard on the hottest days. Many models have a night or quiet mode that limits output.
Our guide on where to put the outdoor unit covers positioning in detail.
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Neighbours, nuisance and planning
Under GOV.UK’s statutory nuisance rules, noise that unreasonably and substantially interferes with the use or enjoyment of a home can be a statutory nuisance, and if the council agrees it must serve an abatement notice.
Planning brings its own noise test. For a heat pump outdoor unit on a house to be permitted development in England, the Planning Portal says it must comply with MCS 020(a), which includes a noise check against the nearest neighbouring window. A cooling-only unit isn’t covered by permitted development, and the council may ask for noise data with an application.
Sounds that mean a fault
- rattling or buzzing that wasn’t there before, often a loose panel or failing fan bearing;
- grinding or squealing from a fan motor;
- a hissing sound near pipe joints, which can mean a leak;
- loud gurgling from the drain, or a pump that runs constantly;
- repeated loud clicking as the unit starts and stops.
Switch off and book a repair visit. Noises tend to get worse, not better.
Making an existing system quieter
Clean filters reduce the fan noise needed to move air. Lower fan speeds and night mode help in bedrooms. Outside, anti-vibration mounts, tightening loose panels and moving a unit out of a corner can all make a difference. A service also checks fan blades and bearings before they get noisy.
What we check at the survey
We look at where bedrooms are, including your neighbours’, the walls the outdoor unit could fix to, nearby reflecting surfaces and the distances involved. We then recommend positions and models with low published sound levels for the rooms that matter most. For bedrooms, see our bedroom guide.
Quick answers
How quiet is a modern indoor unit?
On its lowest fan speed, a good wall unit is very quiet. Check the sound pressure figure for the lowest fan speed on the manufacturer’s data sheet for the exact model.
Will the outdoor unit annoy my neighbours?
It shouldn’t if it’s positioned well, away from their windows, out of reflecting corners and on anti-vibration mounts. We check this at the survey.
What is the difference between sound power and sound pressure?
Sound power is the total noise a unit emits. Sound pressure is what you hear at a set distance. Compare the same type of figure between models.
Is a new noise from my unit a problem?
Often, yes. Rattling, grinding, hissing or constant pump noise can point to a fault. Switch off and have it checked.
Related services
- Air conditioning installation
- Wall-mounted air conditioning
- Air conditioning repair
- Domestic air conditioning
More advice
- Where to Put an Air Conditioning Outdoor Unit
- Air Conditioning for Bedrooms: Complete Guide 2026
- Do I Need Planning Permission for Air Conditioning in the UK?
- Air Conditioning Condensate Pumps: When You Need One
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.