I don't get a "whistling" noise, but wind noise is greater in the X3 than other BMW cars. I think you may be right about the mirror causing some of it. I also think the way the windshield is set into the A pillars instead of a 'flush" fit has something to do with it.

The slight noise does not really bother me at all. It's not that loud. With a few good tunes playing on the stereo, even at low levels, I don't even know it's there.

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'05 X3 3.0i Alpine White/Gray Leather/Alum. Trim
Sport/Steptronic/Servotronic/Panoramic
Prem. Sound/Staggered 18" Style 114 Y-Spokes