When I did front bearings I noticed that I had that exact problem for a couple of months and it stopped. I just pulled the weels, simple green degreaser the wheel and outside of the dust cap and cleaned everything up, and that was that - it stopped. I guess it was a little excess oozing from the dust cap.

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~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net