Any help is appreciated before I send it in to the dealership.

Just started today. My backseat was in stow (folded up position) the past 24 hours. Rattle began after I put the backseat down.

It's driving me insane. Of course google gives all kinds of suggestions, the prevailing opinion in backseat rattles is cabin vents. But since I can stop this rattle by pushing or pulling hard on the seatback, could it be cabin vents?

Another video of the rattle

Maybe try this?

https://www.at4forum.com/threads/rattle-from-backseat.1238/

You might be able to find an acoustic imaging camera (aka "noise camera") to help zero in on the noise source. Vehicle manufacturers have been using them for a number of years now to identify annoying squeaks and rattles. Four or five years ago we started using them in noisy petrochemical plants to help identify the most significant sources of noise among many different noise sources. The image below illustrates how you point the camera in the general direction of the noise source in question and it converts the acoustic energy into a visual color-coded image with the red bullseye being the highest intensity. It takes a while to figure out how to tweak the controls for a given situation, but they actually worked rather well. We tried a couple of different brands; expensive to buy so we just rented them. I haven't kept up with them, but maybe they are more readily available now.

I too have this noise. I've just given it my best effort to ignore it.

When someone's sitting in the backset or when the backseat is up, I cannot hear the rattle. It's not happening all the time now.

I hate to be THAT guy, but these trucks aren't cheap. I think it's crazy to have such rattles in brand new trucks like this. Oh well. Makes me want to get my 2002 stick shift extended cab F150 back.