It is the top rubber bushing on the belt tensioner shock eyelet

There is an easy fix by sliding an O ring in between the bush and the bolt.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1993-Mercedes--Benz-190e--.6-Cooling--System&yearid=1993%40%401993&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6132%3AMBC%7C1486%3AED%7C10000011%3ASM%7C847%40%40190E+2%2E6&catid=240909%40%40Cooling+System&subcatid=241025@@Acc%2E+Belt+Tension+Damper&mode=PA

Search Tensioner Shock .

http://www.peachparts.com/shopforum/showthread.php?t=26091

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A Dalton