Mine did the same thing.. I noticed that there was a little screw loose on the rail. It seems to be where the rail looks like it splits. By the way this happened right after the dealer replaced the rubber web on both sides.. They started to ripped from dry rot I believe... Is your car still in warranty ? Take it back...

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Peter L. House
1998 Sport E430
Azure Blue
110,000k Miles

1987 Oldsmobile Cutlass Salon
V8 Bucket Seats
Factory Moon Roof