Very common rattle on these is from the top bushing on the tensioner damper.
They usually show up at idle , in gear. Have someone do just that as you stick a screwdriver in at the bushing to tighten up the slack. You will see it right away and it can be cured by sandwiching an O ring around the mounting bolt to take up the side to side slack.

Archieves.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1M30LAGSC1TR0Z7Y9Z&year=1986&make=MB&model=190-E-002&category=G&part=Acc.+Belt+Tension+Damper&appEngines=_any