What Causes a Loud Abnormal Noise When Turning on the AC in a Car?

[StNaomi]

When the car's air conditioning is turned on with the AC, the causes of abnormal noise are as follows: 1. Belt Slippage: The air conditioning compressor is driven by the engine crankshaft via a belt. When the belt is loose, aged, severely worn, or the pulley tension is insufficient, it can cause the belt to slip and produce abnormal noise. 2. Air Conditioning Compressor Failure: When the AC switch is turned on, the air conditioning compressor starts operating. If the compressor is damaged, it will produce abnormal noise. 3. Foreign Objects in the Air Duct: If there are foreign objects in the air conditioning system's duct, they can rotate repeatedly with the blower's airflow and collide with the air filter, resulting in abnormal noise.

[Carrie]

I've been driving for over a decade and encountered this situation several times. When the AC is turned on, there's a strange noise - it's most likely an issue with the air conditioning compressor. Worn compressor bearings or loose internal components can produce a metallic clattering sound, especially common in older vehicles. Another possibility is a loose belt slipping and squealing; the sound becomes particularly noticeable when the AC is turned on due to the sudden increase in engine load. The last time my car made such noise, it turned out to be a few leaves stuck in the blower, causing a rattling sound when the fan rotated. Insufficient refrigerant can also cause the compressor to grind dry and produce abnormal noises. These sounds should never be ignored - if the compressor fails, the entire AC system may require major repairs. I recommend turning off the engine, opening the hood to locate the exact source of the noise, and then taking it for professional repair.

[BrodyRose]

Last time, my brand-new car developed this issue just three months after purchase. The AC would sound like a tractor when turned on. After consulting a car-savvy friend, I learned there could be several possible causes: the most common is poor contact in the compressor clutch, which produces a metallic clanking sound when engaging; a worn or cracked belt might screech under tension; or the blower motor bearings could lack lubrication, causing a humming noise that increases with fan speed. Also, check if the cabin air filter is too dirty, as restricted airflow can force the system to work harder. Any of these issues can trigger abnormal noises and indicate a component nearing failure. Don’t tinker blindly—record the sound on your phone and let the dealership technician diagnose it accurately.

[StRiley]

I've studied this issue, and from a mechanical perspective, there are mainly three critical points: First, internal wear in the compressor, such as damaged piston rings, can produce rhythmic knocking sounds. Second, if the AC pulley is out of round or the tensioner pulley is damaged, it will screech during sudden acceleration. Third, air mixed into the refrigerant lines can cause gurgling sounds during circulation. If this happens right after adding refrigerant, it's likely due to incomplete vacuuming. There's also a small chance that the cooling fan blades are deformed, creating clunking noises when rotating. These abnormal sounds are warning signals; continuing to drive could lead to compressor seizure.

[MacCassandra]

Last time my car had this issue, it was quite startling—sounded like metal pieces rattling in a tin can. Upon inspection, it turned out the compressor bracket screws were loose. It was silent at idle, but once the AC was turned on, the vibration increased and caused loud clanking noises. Another classic scenario is when the blower motor bearings disintegrate, producing a sound like sandpaper grinding against metal. If you hear a high-pitched whistling noise, that’s usually the belt slipping. Don’t underestimate these abnormal sounds—they might be linked to the engine pulley system, and in severe cases, can cause the alternator and water pump to fail. If you notice this, immediately turn off the AC and test drive. If the noise disappears, it’s likely an AC system malfunction.