Sound like the bearing is going out. Here's a link for some information about replacing the bearing in our SLK's.

http://www.benzworld.org/forums/r170-slk-class/1165903-just-archives-supercharger-clutch-part-bearing.html

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible