Some really good advice here, What some of the problem is that they bounce a lot and if the rack is not tight in the receiver that added movement only amplifies that bounce.
I have tried almost every Anti Rattle device on the market, most have been no good or so complicated and cumbersome that they are useless. I have a whole tool box full of them.
Until I found the one at Hitch Rider. This will keep everything tight.
Their
Hitch Vice is the best and easiest one I have come across and it really works.