Knocking noise when AC is turned on
 

I have a 99 Saturn SL2 that has a Knocking noise when I turn on the AC. When I turn off the AC, the Knocking noise is gone. Any Ideas?

Hood open engine running A/c off look at the tensioner belt

While your still looking at the tensioner and belt have someone other then you turn the A/C on.

Whats the tensioner and belt doing now?

When the AC is turned on, the belt tensioner pulls about half inch and the belt seems jumpy.

sounds like a locked up compressor. If you have mechanical ability slip the belt off and give the a/c compressor a turn. If the clutch is catching or completely locked up then you will need a new compressor.

Sounds like a good place to start -- keep us informed of your progress.

It was the belt tensioner. I replaced the tensioner and the noise went away. This forum is great and thanks for all your input. Thanks sw2cam!!

Great! Besides, tensioner assemblies are a darned sight cheaper than A/C compressors