It sound strange that BMW have overlook this detail as it would have been tested in wind tunnel and the sound characteristic would have been registered.
I remember reading an article some where ( regarding the wind noise study for side mirror for cars in general) that summarised the cause to the shape of the side mirror and the interaction with the wind from the windscreen side column..something like that.
If we can run the design through a and tunnel, we will probably have a better idea of the effects and maybe design a air deflector to be strategically attached on the car to reduce the sound.
Surely there is a market for this accessories.....