Had identical symptoms on 540i with m62tu engine. The water cooled alternator was the root cause. It has a plastic bushing which centers rear bearing that fell apart causing internal friction and loud noise. I recommend take out serpentine belt and inspect all pulleys / bearings. In my case, alternator caused so much friction that prevented normal idle.

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2012 X5 x35i My new toy
2005 X5 4.4 Her's (Retrofitted Servotronic Steering Rack)
2003 X5 3.0 Mine (Retrofitted Servotronic Steering Rack and Heated Steering Wheel)
2001 M5 Son's car
2000 540i 6sp Sports pkg, supercharged. Son's car (Sold)
2001 530i (Sold)
1999 528i (Sold)
1975 2002 (Sold)