Hi,I seem to have a problem when towing over uneven surfaces? I get a creaking and bumping noise from rear of car,the car is an X trail 2007 I have had the towbar checked by the installer-OK, the car checked by dealer-OK,I have changed the bushes in the alko hitchhead in case of contamination,cleaned the ball until gleaming,it has not made any difference. The caravan is an Abbey 318 2006 1450kgs laden. I would be very grateful for any advice or experiences. Thankyou Denis.