What purges air when you turn key to on?

Mine does it too. When it does, my brake application gauge jumps. My guess is some kind of ABS check.

Also, not sure if you're supposed to, but I don't wait for all those clicks and stuff. I crank the truck right away. Because after it starts, it just does all that stuff again anyway.

 

On newer trucks you want to wait after you turn the switch on to let the computers do their "check downs", before you crank the motor over.
Failure to do so "may" cause communication errors and cause possible issues.
An easy way I did this was to turn the key to the on position before I got into the truck. By the time I got in the seat, enough time had passed for the start up tests to run.

 

Yup, i do this with my bullfrog every time. And in the rare case it doesnt finish by the time i buckle in and fiddle with my ELD it is done.

 

It is the ABS controller cycling the modulator valves. Some call it the chuff test. The purge of air is coming from the modulator valves. If your truck isn't doing it anymore, it usually means your ABS module isn't working, or there's a fault. If you get clicking, but no purge, you may have low air pressure. If you're holding your foot on the brake when they cycle, you will hear the purge from the treadle (foot) valve.