Short bursts of trickling or gurgling as the system starts and stops are normal — that is refrigerant moving through the expansion device. Continuous gurgling, or a new sound that was not there before, is worth investigating.

My neighbour says the outdoor unit is too loud. Where do I stand?

Councils can investigate noise from domestic equipment as a statutory nuisance and, if they agree it is one, serve an abatement notice. In practice, a unit that has got louder usually has a mechanical fault or worn mounts, and fixing that is quicker and cheaper than a dispute.

Can vibration be cured without moving the unit?

Often, yes. Replacing flattened anti-vibration mounts, adding a rubber-isolated bracket set, or moving the unit off a resonant wall panel onto a ground stand solves most of it. Re-siting the unit is a last resort because the pipework has to be re-run and the circuit recovered and recharged.

Low risk

Updated 30 August 2026

Rattling, Gurgling Or Vibrating Noise

The system has become noisy — a rattle from the indoor head, a drone or vibration from the unit outside, or gurgling in the pipework.

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Air conditioning work, illustrative image

What's happening?

Air conditioning noise divides usefully by where you hear it and what it sounds like, and the description narrows the fault more than most people expect. A rattle or scraping from the indoor head is mechanical: the fan barrel out of balance with dust, or the casing clips no longer seated. Gurgling and trickling is fluid — either refrigerant flashing through the expansion device, which is normal in short bursts, or condensate moving in the drain. A hum or drone outside that you feel through the wall is vibration, usually the outdoor unit's anti-vibration mounts flattened with age or brackets bolted straight to masonry. A hard buzz that starts and stops with the compressor is electrical. Noise also matters legally: a persistent outdoor unit can become a statutory noise nuisance that a council can act on.

What usually causes it?

Indoor fan barrel out of balance with a build-up of dust and grease on the blades
Casing panels or filter covers not clipped home, buzzing against the body of the unit
Anti-vibration mounts under the outdoor unit compressed flat, transmitting vibration into the wall
Wall brackets loose, or fixed to a hollow or rendered wall that amplifies the drone
Outdoor fan blade cracked or clipped by debris, throwing it out of balance
Refrigerant flow noise from a restricted expansion device or a low charge
Condensate pump running dry or its impeller worn, producing a rhythmic buzz indoors

Is it dangerous?

Noise on its own is a nuisance rather than a hazard. Two exceptions are worth knowing. A hard electrical buzz combined with heat or a burning smell at the isolator means stop and isolate. And an outdoor unit loud enough to disturb a neighbour is a real problem — councils can investigate noise as a statutory nuisance under environmental protection law and serve an abatement notice, so it is better dealt with as a fault than argued about.

Can I fix it myself?

Narrow it down before you call. Note whether the sound comes from indoors or outdoors, whether it starts with the fan or with the compressor, and whether it changes with fan speed. Press the indoor casing gently while it runs — if the noise stops, a panel or filter cover is not clipped home, and reseating it is the whole fix. Clear stones, leaves or a loose trellis from around the outdoor unit. Anything inside either casing is not yours.

When should you call an air conditioning engineer?

Call an air conditioning engineer if the noise is new and getting worse, if there is a metallic scrape or knock from either unit, or if the outdoor unit is vibrating hard enough to be felt inside the house. Also call if a neighbour has complained: a unit that has become audibly worse usually has a fixable mechanical cause, and fixing it is a better answer than a dispute.

What will the air conditioning engineer actually do?

Run the system and isolate the noise by stage — indoor fan, outdoor fan, compressor, condensate pump
Strip and clean the indoor fan barrel, which is the usual cause of a new rattle or whine
Check and replace the anti-vibration mounts and re-fix or re-site the outdoor brackets
Inspect the outdoor fan blade for cracks and imbalance and replace it if damaged
Take pressures where the noise is refrigerant flow rather than mechanical
Re-check sound levels at the boundary where a neighbour complaint is involved and advise on mounting or screening

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Outdoor Unit Not RunningCaution Water Dripping From The Indoor UnitCaution Musty Smell When The Air Conditioning StartsCaution

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