I think the rattle is something that they wanted to include as a retro feature on these cars.
And yes, I did say cars because I'm not talking about just the Challenger. 06 and 07 Stang owner here and they both rattle all over the place. There are things that are just hanging lose that are meant to be popped out for easy removal of say a door or something similar to that that make noise. I think I've gotten use to it though. Just part of the retro style.