Remove the wheel and check for loose lug nuts, cracks around stud mounting holes, spokes and wheel hub edge. If no cracks are found and the nuts were tight, then check the axle nut for tightness, and up/down play, which would indicate a worn bearing surface on the axle tube.
Remove the brake drum and clean and inspect the bearing. This will mean removing the snap ring and destroying the seal to remove it.
Check to see if the brake drum was rubbing the backing plate. An axle shim may correct this problem.