My C200 developed a whistling/shreaking noise when held on the brake in drive. The sound is present at about 800 rpm to 1,300 rpm. The dealer says it is the belt tensioner. Apparently the hot weather can dry out the bearings in this component. I don't know whether that is right, but he is going to replace the tensioner under warranty next week.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190