i am fairly certain you have some broken welds on the chassis. i had an IDENTICAL complaint on one of my customer's cars that ended up being broken welds that were not visible, but could be verified by pushing around that area with the fender off. i probably spent a week chasing this noise and finally was able to locate it. there is actually an old service bulliten somewhere concerning these broken welds, but i remember it being someplace quite weird, and have been unable to locate it. it ended up going to the body shop to have it corrected, and my customer has not heard the noise since.