Omg the rattle that comes from my back seat or the back wall when my seat is down is unbelievable 2 week old truck and it swords like the back seat is completely going to fall apart.
Nope nothing in the back at all and if I take my fist and tap on the back seat it sounds like plastic on metal all The way across.Do you have any stored in the back seat storage?
It’s something on the back firewall for sure….mine does it too. Wife thinks it's a rear headrest
For a 2 week old truck they better….. and you are correct if I stow it up it reduces it by 90%Stow your rear seat in the upright position and the noise goes away. There is a video on you tube...it is the rear HVAC louver behind the rear seat. Need to pull rear seat per the video but real fix is unclear/unresolved.
I have it as well. I am empty nester....I put my lunch and work bag in the under seat storage compartment and my rear seat is stowed 90% of the time. Note...I just hear it on my driver side...not passenger. I will not take it apart to repair mine and others have indicated that GM dealership will not support under warranty etc. Good luck with your next steps....hopes this helps.