could possibly be your wheel bearing. Does this happen pretty much at any speeds?
also does it pull to one side while braking?
Also here is a pretty easy to way to find out if you have bad wheel bearing
"To check a wheel bearing, grasp the tire at the 12 and 6 o'clock positions and attempt to rock the tire. If you feel any play, the bearings are loose and need to be replaced. Also, rotate the tire by hand. Any roughness or noise from the bearings would also tell you the bearings are worn or damaged and need to be replaced."
http://www.aa1car.com/library/2003/bf50360.htm
also does it pull to one side while braking?
Also here is a pretty easy to way to find out if you have bad wheel bearing
"To check a wheel bearing, grasp the tire at the 12 and 6 o'clock positions and attempt to rock the tire. If you feel any play, the bearings are loose and need to be replaced. Also, rotate the tire by hand. Any roughness or noise from the bearings would also tell you the bearings are worn or damaged and need to be replaced."
http://www.aa1car.com/library/2003/bf50360.htm