Does it happen when you put it in Park and rev the engine?

If not, I would suggest a tranny fluid and Transfer Case fluid change, and see if disappears.

If yes, I'd say the pulleys.

Good Luck!

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01' 3.0i E53 (M54) with 164,000 miles
07' 3.0si E83 (N52) with 134,000 miles
08' 3.0si E70 (N52) with 91,000 miles