Maybe a Cracked Undercarrage Trim Piece
What was causing the noise from my car was a cracked undercarriage trim piece which, when I hit 80 mph, was deflected so it rubbed against the road surface. Maybe that is what you have. Since it was likely caused by me bottoming out the car (easy to do in NYC with our crappy roads), its repair was not covered under warranty, but the trim piece was pretty inexpensive. Why not take a look underneath your car and see if that is the case?