check the drop links on the rear anti roll bar, they make a banging sound when going over bumps, also check that the wheel bolts are tight, Danny1.0 was at the brighton meet, making banging noises from the front, then his wheels fell off. the studs were too short.
mines clunking aswel! ever since i lowered it - i think it may be the panhard rod sliding slighty as i deformed where it goes trying to get the old one out
get to the back of the car and look inside the wheel arches, (easier with the wheels off but not essental) there is the rear axel, and coming off that towards the front of the car, is the anti roll bar, this is connected to the car by a little bar on each side which is about 3 inches long, it has a ball joint on each end. if there is a bit of play (moves even a tiny amount up and down) then that will be the cause. they are a right ###### to get off. i got one off my Lx with two spanners (one to hold it still and one to undo the nut) but the ones on my cabriolet had to be angle grinded off.
they seem like nothing but they make a sound as if the suspention is bottoming out and hitting the bottom of the car, it can be a loud tapping or knocking on the bottom of the car, which should be bias to one side, try folding the seats down to hear it better.