Another belt tensioner question 1998 E320 gas engine
 

The belt tensioner on my E320 is making a loud clacking sound while at idle speed, you can see the tensioner oscillating. As soon as you increase the rpm, the noise goes away and the oscillating stops. Do you all think I may need to replace the tensioner??
Thanks for the information, great site, and great people.

Did you ever figure out what this was? I've got the exact same problem and am wondering the same thing.

It could be the tensioner or just the pulley on the tensioner (the idler pulley if you want to call it that).

Replace the tensioner, it is a crap design and the new part will be an updated version.