It's a quirk as a result of the design on most coupes. Because of the aerodynamics there is really no way to get rid of it. It's catching hold of the wind. the front window opening is so large that it takes in a lot of wind and the seat belt acts like a "whistle". You really can't do anything about it. Although, I have the same problem and have had no success in stopping it myself. I've tried moving the seat around and it did nothing.