whining/whistling from engine bay (video included)

I have a whining/whistling noise coming from engine bay.
it starts after a few minutes of driving. The noise goes away when pressing brake pedal, but immediately comes back. It also goes away when accelerating. When I turn the truck off the noise fades away after a few seconds.

I'm thinking it's either a vacuum leak or a brake booster issue.

Any thoughts what this could be and the best way to diagnose it?

 

? Confirmed that power steering res is full on checkstick and functioning proper

Only thing comes to my brain when I had a similar whine
But that braking goes away I don't recall having that symptom

 

I'll check that. I'm not sure if tapping the brakes would have an effect on the power steering reservoir.

 

Brake booster sounds like a likely culprit, especially since it keeps going after truck is off. If it does it at idle, I’d pull the line to the brake booster, plug where it connects to the intake, and see if it continues.

 

Could be BB check valve

What about doing that 30sec BB test?
Tons of vids out there