It's always tough to diagnose an issue remotely, but my first thought is that this might be a slight Vanos rattle. With the engine running, put your hand on the front of the engine directly on top of the Vanos area. Have someone else rev the engine. If you feel a vibration directly under your hand and if the sound seems to be coming from the top area, then that's most likely the problem. Besian Systems sells a Vanos seal and anti-rattle kit to take care of this issue.

Then again, it may be something as simple as a failing idler or belt tensioner pulley.

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Eric Giles
'04 M3
'07 X5 3.0si
Many others.