I don't even know if this will help but, I had a similar sound in my 91 300E. It turned out to be the dampening shock on the fan belt tensioner. I used an automotive stethascope and just started touching everything under the hood untill I found the noise. I was able to confirm that it was the dampener (it looks like a 6 inch shock absorber) by pushing on the tensioner; this quieted the sound. Be carefull around the belts.