Mine did the same thing, it was driving me nuts! I took it in and had it looked at, first dealer said couldn't do anything about it. Took it to a second and they said the seat back needed to be replaced. They had to special order it, took about a week to get it in. that was about a month ago and so far no more rattle. Mine was replaced under warranty, 2018 3500 CC Dually Laramie.