Belt must be slipping mighty bad. But what the heck, replace it and eliminate some of the mystery. Since it's wearing on one side check that the tensioner pulley bearing isn't on the way out while you're there. When it gets worse you'll really know it.

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Bart

2002 ML500, 2009 CLK350 Cabriolet
93 300D 2.5 Turbo 265,000 Miles (Sold - bad idea)