I had an earlier post about my 1993 300E2.8 with a "ping" at idle when the a/c compressor was on. The ping sounded very much like ignition knock but was only there with the a/c at idle. The post was:

300E Pinging with A/C on

I finally found the source of the problem. It was a defective damper or shock absorber on the belt tensioner. After replacement of the shock absorber, everything is quiet again. It is still amazing to me how much it sounded like ignition knock but I was almost certain it was not from the beginning because it was never present when you would expect ignition knock.

Just an additional note: As I understand it, on the 300 engines up until some time in the 1993 or 1994 time frame, the shock absorber is separate from the tensioner; on later models, it is one piece unit.