Is your hitch a slider hitch? They seem to be a little noisier.
Check all the bolts in the brackets, rails, hitch and pinbox. Then...
If possible have someone else stand outside the vehicle and start/stop gently or until they hear the sounds you are referring.
I made these out of an old bicycle tube and put them between the bed rails and bottom of the hitch. They did help reduce some noise. You could double-up these but it may take more pressure than the 5th wheel being connected. There really isnt as much space as you may think.
