I have worked with the guys at Rock Hard for a long time and I really like them but, I have to say that this is one of the reasons why I have a hard time recommending their rear bumper tire carrier. When running a big heavy spare, there is enough leverage to cause the tire carrier to move around and cause an annoying thumping or rattle. One thing you can do to help mitigate this (on any bumper like it) is to do the following:
1. Remove the spare.
2. Loosen the frame mount bolts.
3. Open the tire carrier so that it sits 90° from the cross member.
4. Lift up the end of the tire carrier with a Hi-Lift so that the entire bumper rotates up as high as possible (shouldn't be too much with the bolts in place).
5. Tack weld the base of the mounting flanges to the frame.
6. Tighten up the bolts.
This will help keep the tire carrier pressed more firmly up against the tailgate and that should help.