Spieluhr...a freind of mine at college told me about something Thule and Saris makes called "Fairing" which is a shield that slopes off the front rack bar and prevents wind from slipping under the bars...
BMW of course doesn't make them but i think the Saris brand fits BMW racks...
however Fairings which seems to be a just simple piece plastic can cost as much as the roof rack themselves.
The Saris Fairing is like $120...forget about Thule prices...
so for now I'd rather take the racks off when not in use.