First... have you been wheeling lately... check to see if any debris caught between the brakes, small pebbles, etc....
Hmmm sounds like it could be your brakes or the rear axle bearings.
Jack up the rear and rotate tire to see if it sounds like the brakes or if need new pads.
If needed remove tire and rotate to get better idea as well. If it's the bearing you should be able to tell if its grinding or loose.