Both sides _could_ be bad clips. The window can get out of position and jam a little slowing it down when the clip is broken.

Best thing to do is remove the door panel and look. If the clips seem fine, then you have more troubleshooting to do.

The clips are really cheap from the dealer (around $1.25). You can get them from Amazon, too, if you don't have a local dealer. Each front window needs two clips.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles