Some things you could try to narrow it down:

Try holding the engine at increasing RPMs, from idle, in park, to see if you can hear the sound. Then, in drive, with the brakes on, increase the RPMs from idle 250 at a time and hold them steady for a short time, to see if you can hear the sound. If you hear no sound from either it is far more likely the sound is associated with something that only moves when your X is moving and we can go from there.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas