I recently purchased a pre owned bmw 3 series. Whenever and only when I turn on the AC the RPM goes to 1 and then drops. It’s also noisy from the inside. When I go to turn it off it also makes noise before shutting off. It only happens with the AC. My previous BMW never did this. Could it be the belt? Or a dirty fuel injector? I’ve been reading on it and it’s giving me several answers. I told the dealership about it and they said it was normal – though like I stated before my other BMW never made this noise.
Compressor and/or compressor pulley probably has bad bearings. Remove the belt and turn the compressor clutch and pulley and check for binding. If you do not know how, take the car to a reputable garage.