I've had my 2015 in 3 times for this same issue. First time they added high temp lube to the flapper valve. This worked for a couple of day's and then the squeak came back. Took it back at my next service interval and they replaced the entire exhaust system with the so called updated part number for the flapper valve. The noise went away but returned 2 months later and when I took it back they stated that GM was aware of the issue but they do not have a fix for it yet. This is the same response I got for the leaf spring noise! They said the only direction that GM gave them is to use the high temp lube on the flapper until a fix was released. WTF!!!