Brakes? Something broken in the suspension or god forbid, the rear end. I had an odd condition with my old Tahoe - the drive shaft would cluck going over speed bumps. I put in new u-joints, checked the rear suspension, nothing obvious. My buddy rode around in the back while cruising over humps - brought it back and greased the yoke/splines and the noise went away. All that said was sometimes things are just not obvious and you need to keep picking at it.
