Bought srt6 at 24K miles. It always had a small shake at idle, and it was definately from backfire, not from cams. It wasn't consistent at all, but could be expected to happen every few seconds. Definately like a pop, a misfire, enough to shake your butt but not the car.

It was an original worry for me, but after replacing spark plugs, fuel pump, fuel filter, and even more, it never went away. Car always ran fine.

I may even suspect a clogged cat as some of the amg-32 mercedes benz, a few ppl on here if you search, had clogged cats replaced. I never took it that far.