not the exhaust I checked. very secure. not spare tire. I don't have one in trunk.im not bottoming out, its when I go over a bump or some rough road which I drive very carefully over, the back of the car underneath sounds like its thumping and not secure or something. thought it was gas tank banging around but its very secure. everything back there all new and secure. just can't figure out what else it could be. very frustrating !