I think I may have fixed the whistling noise! I noticed that it emitted from the passenger side of the cabin and it was in sync with engine RPM. Also it was there when the vehicle is on the move or stationary. So I inspected and cleaned the pressure regulating valves and the cavity where it sits (# 13 in the diagram link below). The diaphragms were not torn but were dirty, especially the passenger side which had creamy stuff. Now no whistling noise!
RealOEM.com - Online BMW Parts Catalog
Take care not to break the tabs on the cover of the valves. I used a small flat-head screw driver to gently unlock the tabs I could reach and then gently pulled the cover off. When installing it back make sure you hear the tabs snapping in as you push the cover in place.
Good luck and hopefully this is also the root cause of your whistling noise.