Q: Rattling noise when the AC comes on

Hey there. There are some obvious possibilities: the fan or blower noise due to motor defect; the A/C clutch is chattering or slipping, and/or the clutch bearing is worn, loose, or wobbly; the compressor has an internal mechanical fault or is low on refrigerant oil due to past refrigerant leaks; the condenser fan is defective or hitting an obstruction; or the serpentine belt is slipping on the A/C compressing pulley. All of these possibilities can be evaluated by a mechanic.

If you want to take a quick look yourself, you can have an assistant sit in the vehicle while the engine is running and the vehicle is in PARK with the EMERGENCY BRAKE fully applied. While you look under the hood from a safe viewing distance, have the assistant turn the A/C system on and off (leave an interval so you are not short cycling the system). If you are lucky, the noise might be obvious and "localized" enough that you can pinpoint it to the compressor, fans, belt or another part that I have mentioned. If you would like to have this diagnosed for you, a certified technician from YourMechanic can come to your car’s location to inspect the A/C system and pinpoint the needed repair.