[Pyrotechnic]

Oh god. I had the same thing happen to me with the gas cap. I an issue with my truck randomly stalling out and not wanting to start back up. When I'd pop the cap off to the fill the tank, I'd get the "hissss" but since all new cars do it I was used to it and I never thought much of it.

Then I realized, my truck was non emissions and the tanks should vent the atmosphere, meaning there should be no pressure at all. I drop the tank and the 5/16 vent tube was clogged SOLID. Turns out that "hissss" was air entering the tank, from the vacuum the fuel pump had pulled on it. That electric fuel pump died not too long after.

Nice truck though and I think you will love that modern drivetrain.

Since you are going to be doing a lot of pulling, I'd recommend installing a spin on filter somewhere on the transmission cooler lines. You just use a oil filter relocation bracket that takes a common 3/4-16 oil filter. The filter in the pan doesn't catch the really fine stuff, but the oil filter will. I believe the Allison and other big automatics use an external filter to increase the transmission life. It's a $30-$40 dollar modification that will make that transmission last a lot longer.