problem with my hitch

I have a problem with my hitch,when I apply the brakes I get a slapping sound and it feels like its come from the hitch.I just traded veh and they installed a new hitch then I stopped at dealer and brought a new shaft and a new 2in ball. I had the same sound fr my old veh I traded in.
I called tm and talked to service tec and was told tm used a self adj hitch and said that he has not heard of this.
Could it be something Im doing wrong?Ive tried loseing the elec brake up some but still hearing sound.: