You are correct those two heatshields on each side of the muffler can rattle at low RPMs. Mine rattled on slight acceleration at low speed or at idle with the A/C on. Looking at it I had about a 1/4 in clearance on the right one and virtually none on the left. I just bent it down to get the desired clearance.
BTW I spent a lot a time on the tailgate bumpers, latch, center brake light and lisence plate but the all time it was that left side heatshield rattling.......maybe mine was just a one off but IMO worth checking before doing anything else