It may be that the body mounting area pulled or broke out. This is for the body mount that is right behind the rear wheel well. Hard to describe, and the car this happened on isn't here any more for picture taking/posting.
What happens over bumps such as speed bumps and concrete slab seam bumps is that the body lifts off the frame then smacks back down onto it. Which causes the sound you are (may be?) hearing. It sure did sound like a bowling ball was being dropped in the trunk each time.
Use a pry bar between the frame and body behind the rear wheel well. See if you can lift the body off of the frame.
In the case of this car a sheet metal circle broke away, both sides. To fix it I cut through the truck floor and the body mount floor. Inside is a captive nut that can be reused. This is an area that is double layered with the trunk floor above and the body mount area below.
Once opened up welded in a U-shaped channel. The bottom of this channel became the new body mount perch area. The welding was done in two steps. First to the lower sheet metal. Then the trunk floor folded back in to meet and that too welded.
Caulked and painted to finish it off.
RemoveBeforeFlight
