I've got a wheel fork rack like this:
which whistles between 60 and 65 MPH. One solution is to tilt it back. I actually tilt it so far that the wheel touches the car roof, which has the benefit of keeping it from spinning.
I don't see any problem with this, do you?
An alternative solution would be to use duct tape to smooth the edges of the fork, so that the wind passes smoothly over it.