[Cheshirelaugh]

I've heard this too, assumed (hoped) it was just a whistle effect from the mirror. Haven't paid attention to what speeds I was doing when it starts/stops.

Also, another possibility (I really haven't looked into it close enough)... My on my old 91 Grand Prix, when the windows were down the seat-belt hinge/pully...thing up above the driver's left shoulder would whistle... Maybe it's something like that. Just throwing that out there as a thought.