There are plastic sliding blocks on some models... e36 comes to mind. But it depends on what type of click you are having. Very faint clicks are normal... it's the motor hitting it's end of travel and possibly triggering a relay or the motor itself makes the noise. Haven't looked into it to see where exactly it comes from since it's normal.

If you are talking about a louder clunking type of click... the regulator could be going out, or was replaced by a PO and not done correctly. Both our fronts make a clunking type of noise and were replaced by the PO. I did the rears myself and there is no such noise. Might pull the panel off and have a gander in there... maybe they didn't use OEM regulators or something?

If you do need a regulator, get a good one. Should run ~$120 - 150.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)