Doesn't sound like a belt to me, sounds more like one of the pulleys is bad. In my experience with the belt is removed a good pulley will also be quiet when you spin it by hand, a bad pulley will be loud, you can hear the bearings 'rattling'.

I don't think the belts have been eliminated. Usually a bad belt will have cracks on the side toward the pulleys and likely feel loose, having stretched. To much belt dressing will cause the belt to slip more for a little while but it should quiet down if the belt is good. If you remove the belts to check pulleys be sure to check the water pump and alternator pulleys'.

Does the sound go away or change after you get to operating temp?

Does it get louder when you turn the steering wheel from lock to lock?

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