What makes my car hum sound at speeds over 45 mph and the sound stops when brakes are applied?

What makes my car hum sound at speeds over 45 mph and the sound stops when brakes are applied?
On the Range Rover Mk III this is a common problem. If the noise is coming from the windshield/dash, its actually the wind hitting the bottom of the windshield -- the trim there is loose enough that Rover issued a TSB on the matter. Make sure the plastic exterior trim fits snug...