LionSeal

That was one of my initial thoughts as well as I have had heat shield issues on previous vehicles. I shook everything that I could shake and verified all bolts were tight and it's still making the noise.

I tried turning off the stop/start, and it still makes the noise when coming to a stop and occasionally when cruising.

I have a 2021 AT4 6.2L that I had the dealer install the GM Performance exhaust. At first everything was fine, but I am now hear a metallic flapping / screeching sound every time the truck comes out of auto stop/start, when I am slowing down to a stop and the truck downshifts, and occasionally at cruising speeds when the gas pedal is feathered. It is a brief noise that doesn't last long but is still like listening to nails on a chalkboard. It is loudest when slowing down to a stop when backpressure from the transmission downshifting releases. It sounds similar to issues people have had with the adaptive exhaust valves, but the performance exhaust eliminates those (unless they go internal with the performance exhaust similar to Ram). I uploaded a couple of youtube videos trying to capture the sound. If you listen carefully you can hear the noise right after the engine turns over: I took it to the dealer who installed the exhaust and was quickly discredited. They said it was the starter and claimed they could not hear it at any other times. - Could this actually be the starter? It doesn't make the noise during a cold start, only when the auto stop/start reengages. - Could it be something with the performance exhaust? Are there adaptive valves internal to the muffler? - Has anyone come across anything like this in the past? Any insight would be greatly appreciated as I don't know how much longer I can withstand this subtle torture.