I would start with the serp belt and belt driven items. Try isolating the sound as much as you can with the car running - be careful!!

Does the pitch of the noise change with engine rpm?

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1996 (W210) E300D 86K - Traded in for a Lexus
1992 (W124) 300D 2.5 Turbo 202K - Sold
1983 (W123) 300D, 146K - Sold
1970 280S, 263K - Sold - Beginning of addiction