look into an anti-rrattle hitch pin. I have this one.

Let's Go Aero (SHP 2080) 1/2" Silent Hitch Pin with Lock

http://www.amazon.com/Lets-Go-Aero-S...+Pin+with+Lock

also saw this one

Amazon.com : Swagman Anti-Wobble Threaded Hitch Pin for Universal Racks, 1/2-Inch : Automotive Bike Racks : Sports & Outdoors

I tried a wedge type device but there wasn't enough room to get the wedge in.
Boone Outdoor aka The Wedge No Rattle Hitch Stabilizer

http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1