If you have an Alko tow hitch with the built instabliser friction pads, then it could be these, which is where I think Sprokets is coming from. Use a proprietry brake cleaner to clean the tow ball and the hitch to remove any grease or oil.If in doubt, get your local caravan dealer to chek it all over.
But as another thought, is the towing bracket fixed to the car as firmly as it should be? This could be moving just a bit as the load changes as you go round corners. Its worth getting the brackets fixing checked, as if they are not tight enough, they will damage the underbody of the car.