Maybe a hub bearing or loose lug nuts, Try jacking up the rear with the trans in PARK and spin the wheel and see if noise is there, If it is check to see if you have side to side play in the wheel- hub, If it has more then just a slight bit of play it should not be driven. Even if there is no play does not mean the bearing is good. if no play i would remove the wheel (you should have a jack stand under car at this point) and drum and then turn the hub by hand, this should turn smoothly and without any noise.