I once had a pen roll down the dash and into the windshield vent slot. This used to slide around the vent duct when cornering - very annoying. I removed the pen by first removing the side window demister vent grill.

Removing the instrument cluster is straight forward should your rattle be something from that area....

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Cheers, Brian

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2001 X5 4.4i Sport
Black Sapphire, Hi Gloss Shadowline, Full Privacy Glass.
Too Cool for Jack Bauer