[Munch]

One suggestion might be to do a tire rotation (move the rear tires to the front; fronts to the back) and make sure the tire pressures are at or a bit over recommended pressure. Then go for a test drive. If the noise moves to the front, you might have a 'cupped' tire. When you get new tires you should get an alignment. If the noise stays at the rear, then you'll need to investigate further whether its a bearing or a brake rotor/disc noise. Good luck!