If you have an alko stabiliser then the noise is probably coming from that. Make sure your towball is frees from rust and dirt, a rub with wire wool, emery paper or such should do it. Same with the pads inside the hitch head. Some people recommend giving them both a clean with white spirits or brake cleaner afterwards. I have tried using white spirits twice but both times made the stabiliser even louder, emery pad had to come back out!
You will find some info on this on Alko's website.
Robert