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?

It sounds like you AC compressor/clutch is making that sound...

Ever figure out what it was? I just installed a brand new compressor in my 300D and had it converted to R134a. I also installed a new belt for the compressor while I was at it. I experience the same knocking sound, but only at idle (~700RPM). If I increase engine speed to close to 1000RPM, the sound promptly goes away. It's annoying and somewhat embarassing, but I suppose I can live with it.. the A/C is nice and cold. The sound goes away as soon as I turn off the compressor.

I still have not figured it out. Your description sounds the same as mine and like you, I can live with it but I would really like to know what it is. I have had the car since new and I do not remember it doing this until the last year or so. Nothing has ever been done to the a/c itself--not even added any R134. I did put a new belt a few months ago. It made the same sound before and after installing the new belt. I guess it could be the clutch but I really do not know how to trouble shoot it.