Rear end noise at 50mph?

Ive noticed a strange noise coming from the rear end, at 50mph (on the dot). The sound cannot be heard above 50mph and only becomes noticeable at 50mph and after deceleration to 48mph (once again, on the dot). Once the speed decreases below 45. The sound can no longer be heard. To describe the sound. Its like when you blow into the top of a bottle and it makes a whir sound. But the sound my car makes is a little sharper, more high pitched. It has been doing it for some time now, and hasn't changed a bit. The sound, or the mph range. And I have also picked up a squeak sound while just rolling and then once I accelerate, the squeak is gone. So I suspect brakes to be my culprit for the rolling squeak. But figured id throw that in there. Id suspect a failing wheel bearing, but it seems odd with a sound that is only noticeable between 48 and 50. I figured with a wheel bearing it would be more constant throughout the mph range and get louder with speed. So my question is. What would cause this sound? Thanks!