Could also be a loose bit of trim on the underside. I had a similar situation with a noise from the rear - it was really noticeable at 80+mph. Thanks to another board, I found that there was aa trim pience on the rear bumper that had not been properly re-installed after I had my phone (pre-bluetooth) and DVD (pre pano roof) added at the dealer. They fixed it and the noise went away.

I must add that having the benefit of this boards and others like it really helps my Service Advisor. He now follows them regularly.

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07 X5 E70 4.8i
04 Z4 E85 3.0