my 2017 5.3l had the rattle also, it would rattle when the AFM would run in 4 cyl and you were on the gas but not enough to switch back to 8 cyl. i removed the flapper valve and never looked back. that was 30,000 miles ago. i had a 2015 5.3l that never had this issue, only difference was on the 2015 the flapper was in front of the muffler, on the 2017 it is after the muffler