Slight knocking sound when car runs over rough ground/road?

Thanks to all who helped me solve that damn leak on the Mondeo door...it was the foam membrane..heres another...have a 2001 Diesel Mondeo, 2Lt Diesel. When I leave smooth road and go onto rough road, a kinda of knocking comes into the car, sounds as if its suspension related, the mechanic who prepared it for its NCT (car test in Ireland every 2 years) couldnt find where it was comming from. It passed the NCT no bother, even with the knocking in it, the mechanic thinks,and only thinks it may be the top part of the shock, but as it passed the NCT he was reluctant to spend his time and my money investigating it further, the car isnt wearing tyres improperly or anything. Just when you go onto a rough surface a "on-off" dum dum dum sound (knocking noise can be heard) drove me mad for the first few weeks I had the car, ime used to it now, Ime getting new tyres and tracking shortly so I said Ide check it out myself...any ideas and if so how to fix etc....Joe