Thanks for the suggestions. Looks like I've got lots of things to test. I'll tear it all apart asap and try to figure out if it's the motor, the drive gear, the regulator, or something else.

Do I have to take the regulator completely out to remove the motor, or will removing the bolts that hold it the to frame without actually removing the regulator give me enough room to disconnect the motor so I can test it?

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT