Hello,
Great site by the way!!
Now to the problem. My 85 190E make this odd noise while idling. A slight increase in RPM's makes it quiet again. While looking at the engine from below, i see the bottem of the serp. belt flopping and the belt tensioner vibrating as well. I think the noise is comming from either the tensioner or the " belt shock" I was told about the shock by someone else, i haven't a clue what a belt shock is. Does this sound like a common problem? Many thanks in advance.

Check you engien fan belt if loose repalce the belt tensioner assembly & tensioner shock & fan belt. if not loose replace the tensioner shock. singal belt drive belt tensioning device alway have is problem.