It is likely some play where the side to side cradle is attached to the base. If like my Reese the cradle is mounted on a bar that is welded to the base. If you get behind the hitch and push/pull on the top hitch plate it will probably slide back and forth just a little. But enough to make a noise when it does.
If you can find large (1 1/4" ID) nylon flat washer, you can cut out a part of one side of it and slip it down over the shaft between the cradle and base. I could not find one locally. I bought a thin round plastic electrical box cover, drilled a 1 1/4" hole in it, slipped it in place, and (almost) no more noise.