Rattling in Jetta 2004

I bought a Jetta GLS about three months back and started noticing rattling when I went over bumps at low speeds of between 10-35 mph right after I bought it. I have taken it to a couple of dealers several times now and they have not been able to find the problem. The rattling seems to be coming from the rear of the car. The dealers tried insulating the door, shifting the position of the fuel tank, etc.

Has anyone had a similar experience with their 2004 or earlier JEtta and what fixed it?

Thanks

It sounds simple, but maybe its your spare donut tire not secured down, or tools back there..? Do you have modified exhaust or suspension? My Jetta used to rattle (modded exh & susp) with 4-5 people in the car--exh pipe was vibrating against the undercarriage. Food for thought.