I also bet you can take it to 10 dealers, and none will find anything.
With something like that, as was mentioned, can be a spot weld loose, the shock absorber, exhaust heat shield, etc.
What I'd do is get under the car, check all critical suspension components, check clearance from exhaust shields, and touch everything around there to try to find the culprit. If nothing, just drive the car. If it gets worse, it'd be easier to find it later on.
As a perfectionist myself, I got to the conclusion if I try to fix every single noise and rattle on every car, I'd just go nuts. Most of the noises we hear are just annoying. I learned to 'tune them out' in my brain... or the stereo. The ones that really bother me, I usually find them myself and take care of them. But something that sounds on a hard bump, I wouldn't even bother.