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.