Drill a hole in the bottom corner of the towbar weld a nut over the hole and screw a bolt into the nut.
Then when you put you reciever in tighten up the nut and thats the end of the noise.
This concept comes standard on most Haman Reese towbars, I'll try and find a pic and post it for you.
I had it on my last towbar, the bolt was drilled through the corner to stop both horizontal and vertical movement.
Link:
http://www.coldys.com.au/towbars_files/Page392.htm
Not the best picture but if you look at the 3 pics on the right, the top one shows the nut and bolt in the reciever on the bottom corner.