I had the same problem on both front doors. Took it to the dealer twice and both times they applied silicone to some rubber seal inside the door panel. Both times the problem stopped but slowly came back after 3-4 months. The third time, they installed a teflon tape of some sort and that did the trick. No more squeaking. Same thing happened to the wife's X5.

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In the garage
2008 Infiniti G37S Coupe (Traded)
2011 BMW X5 xDrive35i
2011 BMW X5 xDrive50i