JLock

I have a 2017 GMC Siearra 1500 with the 5.3L & 8 speed transmission. There is a rattle noise that you can hear at cold start. I have seen posts on this issue and answers seem to be all over the place. When my truck sits overnight or while at work all day, there is a rattle that sounds to me like it is coming from the exhaust. I haven't gotten under the truck to see exactly where the sound is coming from. It typically lasts about 5-10 seconds. If you immediately put the truck in gear after startup the rattle goes away. Any info on exactly what this may be is appreciated. I have seen suggestions of torque converter, heat shield, hole in exhaust, flapper valve, and bad catalytic converter. Can anybody narrow this down to one of these issues?

I have a stock 2017 GMC Sierra 1500 slt preferred package. Time to get new tires and the Mickey Thompson Baja boss at’s look promising. Can anybody tell me if the 275/60 will fit without a level and and real world reviews on these tires (particularly road noise)? It shows that they measure at a 33.4 which is a little bigger than other 275/60s. Pics also appreciated