Just had the same rattle issue. 2019 GMC Sierra 1500 SLT Crew Cab Short Bed with 16,500 miles.
Only happens when it's hot. Sounds like a heat shield is loose. Especially noticeable with the rear slide window open.
It's an internal baffle in the muffler that has come loose. Part number for my repair was 84657793 which is the muffler, pipe and brackets. The part list price is over $800.00.
Mine got worse over time, mine would do it really bad at medium speed, about 45 mph.
To prove it while stationery, get it good and warm and tap the muffler with a rubber mallet.
Mine was covered under factory 36 month / 36,000 mile warranty.
Just had the same rattle issue. 2019 GMC Sierra 1500 SLT Crew Cab Short Bed with 16,500 miles. Only happens when it's hot. Sounds like a heat shield is loose. Especially noticeable with the rear slide window open. It's an internal baffle in the muffler that has come loose. Part number for my repair was 84657793 which is the muffler, pipe and brackets. The part list price is over $800.00. Mine got worse over time, mine would do it really bad at medium speed, about 45 mph. To prove it while stationery, get it good and warm and tap the muffler with a rubber mallet. Mine was covered under factory 36 month / 36,000 mile warranty.