Hissing brakes, is it normal?

I'm pretty familiar with the sound of the booster pump and, as some of you have described it, it's more of a growl than a hiss. It's easy to identify because it runs if the engine is off, where it sounds pretty loud.

This is definitely a hiss. A hiss like a punctured basketball, air escaping from a tire, a ticked off cat, a punctured space ship, etc... It's a pure hiss without any machine sound behind it (besides the normal engine running noise).

Thanks for all the ideas so far. (To the person who suggested I find another mechanic, feel free to point one out to me in my area, but, in the meantime, I'm happy I found an honest shop that does good work, even if I have to push them sometimes. My question here will help me decide if I should push this sound as something they need to look in to.)