I had exactly the sound you describe on a 190E a year ago. If you carefully run dripping water on the belt with the engine idling, it helps isolate the source. I changed the single belt tensioner, cheap and simple to replace. Once the old one was off, I found the centre bearing shot ! Good luck !

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1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)