Car AC makes a loud noise when turned on

Symptoms

• Noise starts right when AC is turned on and stops when it's turned off
• Squealing, grinding, growling, or rattling sound from under the hood
• Noise changes with engine RPM
• Vehicle otherwise runs normally with AC off

Likely causes

• CommonWorn or slipping serpentine/AC beltThe extra load of the compressor engaging can cause a marginal belt to squeal or slip.
• CommonFailing AC compressor bearingA grinding or growling noise that only appears with the compressor running points to internal bearing wear.
• PossibleCompressor clutch bearing wearing outSeparate from the belt, the small bearing inside the clutch pulley can wear and produce a knocking or squealing noise on engagement.
• PossibleLow or contaminated compressor oilInsufficient lubrication inside the compressor can cause metallic noise under load.
• Less commonLoose mounting bracket or boltsVibration from a loose compressor mount can create rattling that's more noticeable under AC load.

What to do

1. Turn the AC on and off a few times with the hood open (engine off, key on accessory if safe, or have a helper) to confirm the noise correlates exactly with AC engagement.
2. Visually inspect the serpentine belt for glazing, cracking, or fraying, and check belt tension.
3. Listen closely near the compressor pulley itself versus the belt path to help localize a grinding/growling sound to the compressor bearing.
4. Check that all compressor mounting bolts and brackets are tight.
5. Stop using the AC if the noise is a harsh grinding sound, since running a failing compressor can send metal debris through the whole AC system and cause expensive damage.
6. Have the compressor, clutch bearing, or belt replaced based on what's identified as the source.

Tools and parts

When to call a pro

See a mechanic promptly for a grinding or growling noise from the compressor, since continuing to run it can send metal debris through the AC system and turn a bearing repair into a full system replacement.

Safety