Worrying clunk when going over bumps

Hi all,

We have a Lunar Cosmos 2012 van, and had several trips with our Honda CRV  '05 without incident. However we have recently changed towcar (Honda CRV '57) and on our last trip to Bath, when driving on uneven roads or going over a largeish dip a clunk comes from the rear. The new towbar is securely fitted to the anchor points on the Honda chassis, and I have used the Alko tow ball that came when we bought the caravan new this year. I have looked for marks on the hitch to see if it was fouling the towball, nothing evident. The rig looks to be level when hitched to the car, could it be the towball is too high / low? Any suggestions would be most welcome.

Thanks

Kevin