When a car air conditioning system produces abnormal noise during operation, it’s often a sign of a developing fault. These noises may originate from internal system issues or external mechanical components. With over 23 years of experience in the automotive A/C parts industry, the Anchor Group team shares a professional guide to help technicians and distributors identify the causes of car AC noise and the correct diagnostic process.
Car AC noise can be categorized into two major types:
External Noise： Comes from components such as the compressor, AC clutch, belt, or blower motor
Internal noise is generated from the AC system’s internal circulation and working components. Common causes include:
Moisture Inside the System
When moisture remains inside the A/C system, it often creates a sharp “hissing” noise near the expansion valve. This happens as moisture freezes or disrupts refrigerant flow. To fix this issue, replace the receiver-drier, pull a deep vacuum to remove moisture, and recharge the system with the correct amount of refrigerant.
Excessive High-Side Pressure
If the high-side pressure is too high, the compressor may run roughly and produce vibration or abnormal noise. Poor condenser heat dissipation or air trapped in the system is usually the cause. Check the condenser’s cooling performance, bleed any trapped air, and adjust the refrigerant charge to bring the pressure back to normal.
Low Refrigerant Chargμι
A low refrigerant level often causes a distinct “hissing” noise at the evaporator inlet. When this occurs, the system should be inspected for leaks and repaired as needed. After repairs, recharge the refrigerant to the specified level to restore proper cooling and eliminate the noise.
Overcharged Refrigerant
Too much refrigerant can cause the high-pressure lines to vibrate, resulting in a knocking sound from the compressor. The solution is to release the excess refrigerant until the system pressure falls within the recommended range. Once corrected, the noise typically subsides.
Below are the most common causes of external AC noise and how to troubleshoot them.
Compressor Belt Noise
A squealing or slipping noise when the AC is on is usually a sign of a worn or loose compressor belt. This is one of the most common causes of car AC noise. Replacing the belt generally resolves the issue. It’s also important to check belt tension—press down at the midpoint of the belt and ensure the deflection is between 10–15 mm. Proper tension helps prevent noise, slippage, and reduced AC performance.
A/C Compressor Clutch Noise
The AC compressor clutch is another common source of unwanted noise. Drivers may hear grinding, metal-knocking, friction, or intermittent squealing when the clutch engages. In many cases, the noise is caused by loose pulley mounting screws, a damaged shaft key, or worn clutch components. Tightening the screws, repairing the key, or replacing worn clutch parts can solve the problem. A failing clutch bearing can also create noise and typically requires replacement. Sometimes the issue is due to an excessive clutch air gap; adjusting the air gap to the manufacturer’s specification often eliminates the noise.
Internal AC Compressor Noise
If the noise is coming from inside the AC compressor itself, the issue is more serious. Loose mounting bolts or a damaged bracket can cause vibration and rattling, and should be tightened or replaced. Internal knocking or abnormal compressor noise may point to worn pistons, damaged valves, or internal component failure. Faulty discharge or suction valves often require replacing the valve plates or valve assembly. If internal damage is suspected, a proper inspection through the high-pressure service valve is recommended. In many cases, repairing or replacing the compressor is necessary to restore quiet and efficient AC performance.
Blower Motor (Fan) Noise in the Cabin
If you hear noise coming from the AC inside the car, the blower motor is often the culprit. A deformed or damaged fan wheel can cause the AC fan to become noisy and may require repair or full blower motor replacement. Worn blower motor bearings can also create a whirring or grinding noise. Repairing or replacing the blower motor usually restores quiet air circulation and improves cabin comfort.
Ως professional supplier of automotive air conditioning components, we provide:
✔ OEM & aftermarket AC compressors and clutch kits
✔ AC clutch, pulley, bearing, receiver drier, expansion valve, blower motor, and core parts
✔ One-stop product matching & technical support for global distributors
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