Wouldn't hurt to put some silicone spray in there, pretty sure thats what i used on my old 4.8is one winter with the same issue. While your at it, spray the corners up and down of your window tracks as well and roll the window up and down a few times.

Helps keep them run smooth, i had a regulator that was running pretty slow and has since been fine for 2+ years. I do my tracks every 6 months or so.

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2006 X5 4.8is Fully Loaded