This was addressed on another board that I frequent as well. I have and have had a similar issue with my receiver. The rattling comes from the fit between the draw bar and the receiver. This fit is sloppy to assure that when a draw bar is left in the receiver of the hitch assembly, it does not rust together. While it is noisy and drivces me crazy, I accept the fact that I won't have one rust tight and not be able to be removed.
By all means check to make sure that the ball and tongue are both sized correctly. Quick to check and a major safety issue.
I was planning on making up some type of plastic wedge that could be pressed, by hand, into the gap between the draw bar and the receiver.
Hope this helps.