Have you had a human curl up in back, in the hatch while you go over an offending bump? I had a friend who had a similar problem and had his wife in back and then he did the same with his wife driving. ( both in 50's) No luck. Then he had his 20 year old son go back and he found it, he could hear better and pin pointed it. It ended up being a large wiring harness connector to the back, behind the rear plastic shell, just hanging at the right point hitting the rear wheel well shell that was acting as a drum, He wrapped with foam and tapped it the wheel well with Gorilla tape.
Did you check for stuff under your front seats?
Get some Meguirs/armor all interior spray for plastic and go nuts spraying cracks where interior plastic meets everything. All door and hatch gaskets.
On the rear hatch, there are 2 rubber nubs that let you adjust the rear hatch, close height. Extend them. Spray armor all on your lower and upper suspension rear spring seating spots. Pull the rear seat lower cushion, armor all/vasaline the metal tab points.( push the front back hard pull up). Spray the back seat hinges and where the rear seats met the plastic interior. Spray the hatch floor where the plastic interior touches the hatch floor.