Whenever I go over a small abrupt bump the rear of the vehicle makes a loud thumping sound. I've read other posts regarding noisy bushings, but my problem doesn't fit the description. My bushings don't sqeak. My van is the 159 wb 2500. Is it because it's a heavy duty suspension and my van is essentially unloaded? (400 lb campervan conversion.) I have adjusted the rear cargo doors numerous times with zero results.