Wanted to hopefully help out others that have experienced this issue. Over the past 2 months we have noticed an extremely annoying drone under load in our 2021 Acadia Denali 3.6L. It went away when it was in park, but was really noticeable under load at lower RPM's while accelerating. After having my wife leave it in drive and crawling underneath, I could hear something rattling in the main muffler (right behind the cat's). I was quoted between $2000-2500 to have the entire exhaust system replaced, since it's one welded assembly from cat-back. Being out of our 60k powertrain warranty I thought we were at a lose. Brought it into a reputable local shop and they searched for hours to find a OEM style muffler that would fit to no avail. I have a good relationship with our service team at my local GM dealership, and they were able to cover replacing the exhaust system under the 80k emissions warranty considering the catalytic converters were part of the system. Big win!!
For those out of the 80k warranty, I found a thread on FB that someone ended up finding an aftermarket muffler that fit, which was brought into Pro Muffler + Performance (14022 US-2, Airway Heights, WA 99001). Prior to my GM dealership agreeing to cover it, I went down this rabbit hole and they contacted me with their supplier who had a muffler that fit. Unfortunately I didn't write down the PN when I called as I already had an appt with the dealer setup. However, it seems there is a low cost option for an OEM style muffler out there. Hoping others can use this for this common issue under PIT5863.
MC-10204438-9999.pdf