mate, ive got a totally standard back box and mine knocks if i put people in the rear.. its a nitemare.. i think its my shox are toast though
if it only happens when u put people in the back, id be more inclined to say its your shocks sagging and hitting the bump stops.
if your exhaust is too close to the body work, it will knock as it vibrates regardless as to how many people are in the car.
irmscher: i think your stuck with it mate, unless u can get smaller rubber hangers to bring it closer, its dependant on the manufacturer and how the box was made, ive got a sebring on my mk3 and the pipes sit right up against the cut out on the bumper, and the box has never knocked!!! luck of the draw i think as there doesnt seem to be a lot that can be done about it.