[Lord of Hog]

What sort of thump is it? Thumps from large objects like exhausts hitting the underside make quite a distinctive noise which is quite different to the sound you get from the suspension with a broken spring. This in turn is different to the sound of a knocking shocker. I know it's very difficult to explain verbally. Also does it do it when going over a particular type of bump? Big ones? Sharp ones? Speed hump size ones? Try getting an assistant to sit in the back with a hand on the floor and see if he can feel where it's coming from.