Turn up louder so you can't hear the latches rattle. LOL! I have the same problem with my 6U Gator lightweight rack. All the parts on that thing rattle and it's still in likke-new condition. I have tried (and still use) a 1/4" thick, high density foam pad b/t the rack and the 212 cab it sits on. It has minimal effect, to be honest.

I'm considering buying a traditional wooden rack in hopes that the extra weight will offset/absorb the vibration, but I'd hate to spend $150-someodd bucks for nothing.
I have one of those tilt-back combo amp stands that my rack will fit on, but that's one more thing to carry/setup, takes up more room and just doesn't look as cool as the rack sitting on the cab does.