To me, it sounded like a clunking in a wheel well. After an underbody check, I thought it might be the running boards, the mud flaps, etc. Eventually I gave up and the mechanic took a ride with me for less than a minute, and he smiled and said "it's your anti-roll bars." I've known these mechanics for a while and I know they wouldn't screw me over. In fact, everytime I have work done on my car, I'm in the shop (probably breaking safety rules) chit chatting with the mechanics while they work on my car and the other cars the other mechanics are working on.

It is the sway bar. The reason you/the mechanic, can't see the sway bar as being damaged is because the sway bar breaks at the flex point which is covered by the bushing.