My 91 300E used to do this, when I first got it. Only happened in the rain. Would usually happen when transmission downshifted. It was cured when I properly tightened the belt tensioner to the correct tension. Didn't need to change any parts - just adjust properly.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280