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Old 08-02-2006, 09:31 AM   #12
Registered User
Join Date: Mar 2006
Location: NJ, USA
Posts: 47
My Ride: 98 328is, 06 330ci
Originally Posted by Canuck328i
I get a knocking sound at low RPM from the AC compressor - AC still works, but it makes one hell of a whacking sound at 800 to 1200 RPM. It will only happen when the AC button is on, so if you hear the noise try turning off the AC to see if it goes away.
Sorry for the off-topic reply (don't want to hijack the thread) but if you are still experiencing this, check to make sure your aux fan turns on no later than about 15 seconds after you hit the snowflake key and the compressor turns on with the car stopped at idle.

If the fan does not turn on, particularly after a couple minutes at idle, the head pressure in the A/C system is operating well above normal and that's causing the valves in the compressor to chatter. Aside from poor A/C performance (warm air out of the vents while stopped at a light or whatever), you take a real risk in damaging the compressor.

You should get that checked out soon. In the interim, turn off the A/C when driving below about 15 MPH or while in stop-n-go traffic. As soon as you get past 15 MPH, ram air does the job of the aux fan and the A/C should operate normally.


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