"Rattle Noise Problems on 1999 BMW 328i Air Conditioner"

Silver 1999 BMW 328i Front View

I have a 1999 BMW 328i sedan with automatic climate control. If I turn on only the fan, it makes absolutely no noise and drives perfectly. As soon as I turn on the AC, there is a loud rattle coming from the back center of the engine. It does this upon accelerating and deceleration. If the car is sitting still with the AC on, it makes no rattle. It only rattles at a certain RPM range. If I turn off the AC, the rattle goes away. I know it isn’t the blower motor, I’ve already checked that. I don’t believe it is the compressor either. I’m thinking maybe it is the Idle Control Valve flapping back and forth? Any ideas?
Answer: The idle air control valve or even the DISA valve have no relation to AC operation and will work and thus be noisy on their own without help from the AC. Perhaps you are talking of premature detonation/pinging due to bad gas, or being undergear (if with a manual transmission), or underrev ? If not, then to rule out that it is the AC compressor, turn on the AC while parked and rev the engine a couple of times. If there is no noise then it is not the AC compressor as you suggest. I would also suggest putting up the car on a hoist and check for loose or broken tranny/engine/etc. mounts. Check also the flex joints of the propeller. Good luck.