2010 GMC Terrain Vibrating noise

We have been plagued by this vibrating noise, as best I can explain it. It does not vibrate but sounds like it, much like the noise made when you have the rear windows open while the car is moving. So hard to explain! It only makes the noise at 13-15mph, 23-25mph, 33-35, at its worst at 43-45mph. As long as you maintain that speed, it will continue, only stopping at a different speed. Our dealer has not been able to fix it and we are getting extremely frustrated. They have contacted General Motors but every suggestion they've given has proved not to eliminate the problem.