It turned out to be a bad front wheel bearing. The banging was was the sound of the bearing in the hub assembly failing. At first, the failure was not bad enough to show any play in the front bearing. Yesterday I heard the distinct sound of a spun bearing, so we put it up on a lift and checked the bearing again - the bearing totally gave out and literally was sliding in an out about 1/4 inch the second time it was checked. Mystery solved.