2012 Silverado 2WD, LT, LMG engine, All stock. 84,000 miles. A Florida truck, zero rust.
My very first thought, a plugged up cat. When it happened to my old 79 Impala wagon 305 the muffler shop removed the pipe just after the cat to disconnect the muffler. Started it up and it sounded like the muffler was still attached. A year later I had to rebuild the heads. #8 plug would fowl in a day or two, all exhaust guides were worn and one head was warped.I'm going to blame a plugged exhaust, probably a bad cat.
The primary cause that kills a catalytic is unburned fuel. If this 30 year old truck engine is up in mileage and never touched, these old 350 engines were rock solid, but parts do wear out. If the exhaust is blue, especially on a cold engine, it's oil passing the rings, white smoke is oil passing the exhaust guides and burning off.