I get this exact sound at that exact speed area in my e90 335i msprt. Annoying as hell. Took it into dealer and they said they will not be test driving my car above the legal speed limit to diagnose my car.
The foreman stated he would not break the law to fix my car. At 65-70mph it's dead quiet.
Sounds like a faint humming I thought it was the aerodynamics of the m bumper but some hav said it's the rain gutter and to plug it with a piece of chewing gum. I did that but no luck.