Car air con making noise? Squealing, clicking, rattling or hissing

Find the noise you hear in the table. Below it we explain each noise. The table gives a first clue, not a diagnosis.

A first clue based on the noise. The cause only becomes clear from a check in the workshop.

How do I know whether the noise comes from the air con? You can test it like this:

1. Park the car outside, in P or in neutral, with the handbrake on.
2. Let the engine idle and switch the air con on.
3. After half a minute, switch the air con off and then on again.
4. Does the noise stop or change when you switch the air con off? Then it probably has to do with the air con or the way it is driven. If the noise stays the same, the cause is more likely somewhere else, such as the engine or the exhaust.

Squealing or whining when the air con switches on usually comes from a slipping drive belt. The auxiliary drive belt (also called a serpentine belt or fan belt) is the belt at the front of the engine that drives the air con compressor. When you switch on the air con, the compressor asks the belt for extra power. According to the ANWB, the Dutch motoring organisation, you hear a whining noise when the belt starts to slip, because it is worn or not tight enough.

You often hear it on a cold start, when pulling away or at the moment the air con switches on. The air con usually still cools. Do have it checked if it comes back or gets worse. Sometimes the belt slips because the compressor is running stiffly. A slipping belt wears faster.

According to the ANWB the same belt also drives the alternator, sometimes the water pump and, with hydraulic power steering, the power steering pump. If the belt has small cracks or frayed edges, the ANWB says it needs replacing. Electric cars and most hybrids work differently. There the compressor has its own electric motor and no belt.

Sometimes the belt is fine and a pulley or the tensioner is worn. In the workshop we turn the belt by hand, look for small cracks and shiny glazed patches and feel whether the tensioner and the pulleys have play or turn roughly. We only replace what is worn.

One short click when the air con switches on is normal in many cars. That is the magnetic clutch engaging the compressor. Many newer compressors and electric compressors have no magnetic clutch. Then you hear no click at all.

Does the air con keep clicking or keep switching on and off for short periods? Then something is wrong. Often there is too little refrigerant in the system, for example because of a leak. The pressure sensor then keeps switching the compressor off to protect it. According to the parts manufacturer HELLA, a shortage of refrigerant also causes compressor damage. Pressure that is too high, a sensor, the control valve, the magnetic clutch or wear inside the compressor can cause clicking as well.

So have it measured before any refrigerant goes in. During a diagnosis we measure the high and low pressures, read the fault codes and check how the compressor is controlled. Does the compressor not switch on at all any more? Then read our article on an air con compressor that is not engaging.

A metallic rattle that changes when you switch the air con on or off can point to wear in the compressor, the pulley or a bearing. The compressor is not the only possible cause. A tensioner, a loose bracket or a heat shield can sound the same. That is why we first find out where the noise comes from, with the air con on and off.

Do not wait. If a compressor is damaged inside, metal particles get into the air con system. According to HELLA, contamination of the system must be assumed after a compressor failure, and the system has to be flushed when a new compressor is fitted. The repair then becomes bigger. According to the ANWB a new compressor including flushing the system can cost up to about € 1,000.

We discuss in advance which parts come with a new compressor, based on the manufacturer’s instructions. You get a quote first. Read more on our page about air con compressor replacement.

If the air con hisses briefly after you switch off the air con or the engine, that is usually normal. If it hisses for a long time under the bonnet and the air con cools less and less, there may be a leak. A short hissing or gurgling sound after switching off happens because the high and low pressures in the system even out. How long it lasts and where you hear it differs from car to car.

A soft hiss behind the dashboard while the air con is on often comes from the expansion valve. That is the valve where the refrigerant goes from high to low pressure (see how car air conditioning works). This is usually normal too. Does it hiss under the bonnet, does the hissing last a long time and does the air con cool less well? Then there may be a leak. A leak often starts at an O-ring, a hose, the condenser or the evaporator. Read more in car air con pipe or hose leaking: repair or replace?

We look for a leak with a nitrogen test. We recover the refrigerant, put the system under pressure with dry nitrogen and check whether the pressure drops. According to HELLA this mainly finds larger leaks in places that can be reached. If a leak is hard to find, we also use UV dye and first discuss the next step with you. The European directive on car air con systems (2006/40/EC) says a garage may only refill a system with an abnormal leak once the repair is done. So we do not refill a system we know is leaking. Read more on our page about air con leak detection.

Humming or buzzing behind the dashboard usually comes from the blower fan. That is the fan that blows the air through the vents. Does the noise get louder at a higher fan speed? Then the blower fan is the likely cause. Sometimes a leaf has got into the blower fan or the motor bearings are worn.

Buzzing at the front of the car often comes from the radiator fan. It switches on when the air con is on, also when the car is standing still. That is normal. Have it checked if the noise is new or clearly louder. Do you hear humming under the bonnet, but only when the air con is on? Then it may be a compressor bearing. According to HELLA, too much refrigerant, pressure that is too high or poor airflow through the condenser can also make the compressor noisy. A faulty expansion valve can also buzz, whistle or hum, according to HELLA.

Whistling from the vents often points to a clogged cabin filter (in Dutch) or to a flap in the air ducts that does not close properly. According to the ANWB, replacing the cabin filter at a garage costs € 40 to € 125.

Do you drive an electric car or a plug-in hybrid? Then you will sometimes hear the electric compressor buzz, also when the car is parked or charging. The air con may be cooling the battery. That is normal. Read more on our page about air con for electric and hybrid cars.

If the air con cuts out after a few minutes, for example after ten minutes of driving or in a traffic jam, a safety switch is usually turning the compressor off. Often you also hear repeated clicking. These are the most common causes:

Does the air con cool while you drive but not in traffic? Then we first look at the radiator fan and the condenser. We measure the pressures and read the fault codes before we replace or refill anything. Does the air con work some days and not others? Read car air con not working. If it does cool, but not enough, read car air con not cold or blowing warm air.

Stop straight away in a safe place if you hear a loud, sudden squealing or rattling noise together with one of these signs:

The drive belt may have snapped or come off the pulley. In many cars that belt also drives the alternator and sometimes the water pump or the power steering. Without the alternator the battery no longer charges. Without the water pump the engine can overheat. Read more about an engine that is overheating.

The cost depends on the cause. A belt or a filter is a small repair, a compressor a big one. That is why we first check where the noise comes from. We tell you in advance what the diagnosis costs. After that you get a quote with the parts, the labour and the VAT. We only carry out repairs once you have agreed.

Figures marked “according to the ANWB” are ANWB market guide prices, not a quote from us. Our air con service includes recovering the refrigerant, vacuuming, the leak tightness check, refilling to the factory amount and VAT.

You can read more about prices per part in our article car air con repair cost in the Netherlands. We only replace a compressor when the measurements show it is needed. For other faulty parts, see our page on air con repair. Is the system free of leaks and does it only need refrigerant? Then read about an air con regas. We hold an F-gas certificate for R134a and R1234yf, and our technicians are GMTO-certified (a Dutch certificate for safe work on hybrid and electric cars). Everything about servicing and the air con service is on our page about car air conditioning in Rotterdam.

No. Usually it is not serious straight away. The drive belt slips briefly, or a pulley or tensioner is worn. Have it checked if it comes back, lasts longer or gets worse, because in many cars the same belt also drives the alternator and sometimes the water pump.

One short click when the air con switches on is normal in many cars. If the air con keeps clicking or keeps switching on and off, there is often too little refrigerant. A sensor, the control valve or the magnetic clutch can also be the cause. Have it measured before any refrigerant goes in.

Yes. Usually that is the pressure in the system evening out. It only lasts a short time. If it hisses while you drive, keeps hissing for a long time or the air con cools less well, have the system checked for a leak.

No. Do not drive further than you need to. Switch the air con off and have it checked as soon as possible, because the pulley and the bearing keep turning with the engine. Stop straight away if you also see a warning light, the steering becomes heavy, the engine temperature rises, you smell smoke or the noise gets worse quickly.

Humming behind the dashboard usually comes from the blower fan. Buzzing at the front of the car often comes from the radiator fan, which switches on when the air con is on. That is normal. Have it checked if the noise is new or clearly louder, or if it hums under the bonnet as soon as the air con switches on.

Yes. Often it is. In many electric cars and plug-in hybrids the air con system also cools the battery, and while charging you hear the electric compressor and the fan, even when the car is locked. Does the compressor make a rattling or grinding noise? Then have it checked.

Yes. For most noises that is sensible, but it depends on the noise. A click or a short hiss after switching off is usually normal. Have squealing checked if it comes back and have rattling checked as soon as possible, even if the air con still cools.

Gurgling shortly after switching off the air con is usually normal. If you hear it all the time while the air con is on and it cools less well, there may be too little refrigerant. Water sloshing when you pull away or go round corners often points to a blocked condensate drain. If there is water in the footwell, read our article on a car air con leaking water.

Tell us when you hear the noise: when the air con switches on, while driving, when stationary or after you switch the engine off. Also tell us whether it changes when you switch the air con off and whether the air con still cools. A short recording on your phone helps too. Have your registration number ready.

• ANWB: V-snaar of multiriem piept? Dit moet je doen (in Dutch)
• HELLA: How the air conditioning compressor works
• ANWB: De airco van mijn auto werkt niet goed: wat nu? (in Dutch)
• ANWB: Interieurfilter vervangen (in Dutch)
• EUR-Lex: Directive 2006/40/EC relating to emissions from air-conditioning systems in motor vehicles (Article 6)
• HELLA: Car air-conditioning leak detection