I just had my car in the shop this week to fix this same exact issue. There is a part called the Auxiliary Fan which seems to go bad in the X5s. Not sure about your car, but the item IS covered under the CPO warranty if you have it. Otherwise, the dealership told me that it would be approximately $1,000 part to change out. $500 for part and then 500 for labor.

Anyways, for your reference the write up on the service I received is as follows:

Part Number: 64-54-6-921-381 (Compression Shroud With Fan:645018)

My X is a 2002 3.0i

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Previous: 2002 X5 3.0 / 121,000 miles
Current: 2007 X5 3.0si / 93,000 miles