Well, the outcome wasn't good, replacing the rear seat bracket didn't work. Problem is the rear seat behind the drivers seat, up in the corner of seat back nearest the window (where the black handle behind the seat is) there is a ton of play, that's what is causing the rattle. I folded up an old hand towel and stuffed it back there, quiet as a church mouse.
Service dept said they are leaving the ticket open and reaching out to GM. They seem to think it's inside the seat, I think it's the brackets behind the seat. Is it just me, after spending nearly 70K on a vehicle, I have to resort to jamming old rags in my rig to fix problems. It's fixed, yet....