1993 300E2.8, 66K miles. When the air conditioner compressor turns on at idle, there is a slight knocking/clicking/pinging sound that almost sounds like ignition knock. Not too loud--you can only here it with the window down. As soon as the a/c compressor turns off, everything is quiet. Also, with a slight increase in engine speed, the sound goes away even with a/c on. I have never heard it at normal driving rpms, just at idle. Any ideas on what it could be?