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Noisy air conditioning: what the sounds mean

Updated 23 October 2026 · SEO Dons Editorial

Which air conditioning noises are normal?

More than people expect. Both manufacturers whose manuals were checked for this guide publish lists of sounds that are not malfunctions.

SoundWhat the manufacturer says causes it
Hissing, or a gas-escaping sound, at start, stop or defrostDaikin: refrigerant gas flowing, or refrigerant flow stopping or changing. Mitsubishi Electric: the flow of refrigerant being switched.
A constant low hiss in cooling or defrostDaikin: refrigerant gas flowing through the indoor and outdoor units.
Flowing water, or "choro-choro"Refrigerant flowing in the indoor unit or pipework. Daikin adds that a small flow is kept moving in an idle indoor unit so oil and refrigerant do not stay in the system.
A "zeen" after power-onDaikin: the electronic expansion valve starting; the volume reduces in about a minute.
A low "shah" in cooling or at restDaikin: the optional drain pump running.
Cracking or "pishi-pishi" squeaksParts expanding and contracting with temperature, according to both makers.
A brief buzz as the outdoor unit startsMitsubishi Electric: the sound of the outdoor unit starting.
Ticking from the outdoor unitMitsubishi Electric: the outdoor fan controlling the airflow.
A changing tone from the outdoor unitDaikin: a change in operating frequency.

The pattern is that normal sounds are tied to an event: starting, stopping, defrosting or a load change. A sound that is new, constant or louder than it was is not covered by these lists.

What does hissing mean?

At the start or end of a cycle, it usually means refrigerant moving, as the table shows. Outside those moments it needs a closer look. The retained leakage-checking regulation lists abnormal noises or vibration, ice formation and insufficient cooling capacity among the situations that constitute a presumption of leakage, and says any such presumption must be followed by a direct examination for leakage.

So hissing on its own at a start or stop is not a concern. Hissing that is new, louder or continuous, or that arrives with weaker cooling, ice or oily residue, is a reason to call an F-gas engineer. The refrigerant leaks guide covers the signs, the checks and the legal duties.

What do rattling, buzzing and vibration mean?

Start with how the unit was installed. Daikin warns that running an outdoor unit with its compressor transportation stay still fitted may generate abnormal vibration or noise, and says the unit should sit level on a sufficiently strong base to prevent vibration and noise. Its commissioning checklist asks the installer to check the unit is properly installed to avoid abnormal noises and vibrations at start-up. A rattle that began with a new installation or a relocation therefore goes back to the installer.

Wear and faults come next. Daikin's code list has separate entries for indoor and outdoor fan motor faults, but the manuals checked list only which sounds are normal, not a sound-by-sound fault diagnosis. Engineers work from readings and codes. A buzz that carries on, or comes with tripping, goes beyond the start-up buzz Mitsubishi Electric describes. Daikin says that if a fuse, breaker or earth leakage breaker keeps tripping, the main power supply should be turned off and the dealer contacted, and Regulation 16 of the Electricity at Work Regulations 1989 bars anyone from work needing technical knowledge unless they have it or are suitably supervised. Daikin also warns users not to put fingers or objects into the air inlet or outlet, or to remove the fan guard, because a fan turning at high speed can cause injury.

What about gurgling and water sounds?

Mitsubishi Electric lists flowing-water and occasional hissing sounds as normal when refrigerant or water is flowing in the indoor unit or pipework, or "chugging". A drain pump running is also normal. Gurgling that comes with dripping, staining or an overflow is a drainage problem; the leaking water guide covers the causes.

Can a compressor be diagnosed by its noise?

Not reliably. The leakage-checking regulation lists the parameters analysed when a system misbehaves: pressure, temperature, compressor current, liquid levels and recharge volume. Daikin's code list includes compressor lock and over-current entries alongside an unbalanced inverter supply voltage, which shows a compressor code can have an electrical cause. A noise is a reason to ask for readings, not a verdict. How replacement decisions are made is in compressor replacement and the repair-or-replace test.

Does a loud plant room raise health and safety duties?

It can. HSE says the Control of Noise at Work Regulations 2005 set 80 dB(A) as the daily or weekly average exposure at which employers must assess the risk to workers' health and provide information and training, and 85 dB(A) as the level at which hearing protection and hearing protection zones must be provided. The regulations protect workers' hearing and, HSE says, do not cover nuisance noise, which is the territory of the BS 4142 guide linked below. A unit that has become much louder than it was is worth reporting for this reason as well as for the fault.

Which sounds are urgent, and who should be called?

  • Unusual noise with a burning smell: Daikin says to stop operation and shut off the power if anything unusual occurs, because leaving the unit running may cause breakage, electrical shock or fire.
  • Repeated tripping with a buzz: a competent electrical engineer, after the supply is switched off.
  • Abnormal noise with ice, oil staining or weak cooling: an F-gas engineer.
  • A rattle since installation or a move: the installer.
  • Complaints from neighbours: a planning and environmental health matter, covered in the guide to air conditioning noise limits and BS 4142.

When calling, note when the sound happens, whether it follows starting or stopping, and any fault code. The commercial air conditioning repair page explains how faults are diagnosed, and readers who want a contractor to attend can send an enquiry through the site's form.

Sources

Common questions

Is it normal for the outdoor unit to make a noise after it stops?

Daikin lists a hissing sound immediately after stopping operation or a defrost as the noise of refrigerant caused by flow stop or flow change. It is expected for a short time after a stop. A hiss that carries on or comes with weak cooling should be reported.

Why does the outdoor unit change pitch?

Daikin says a change in the tone of operating noise is caused by a change of frequency, and that outdoor fan speed is controlled during operation to optimise performance. A tone that rises and falls with load is not in itself a fault.

What should be noted before calling about a noise?

Daikin asks callers for the complete model name, the manufacturing number, the installation date, and the symptoms with details of the defect. Add when the sound occurs, whether it follows starting or stopping, how long it lasts and any fault code.

Why does a newly installed outdoor unit vibrate?

Daikin warns that abnormal vibration or noise may be generated if the unit runs with the compressor transportation stay still fitted, and says the unit should sit level on a sufficiently strong base. Both are installation points to raise with the installer.

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