I'm referring to the vacuum booster which is bolted to the firewall of the car under the hood. It is what your brake master cylinder is attached to. Looks like a round drum sort of shape, typically black in color. The sound is air escaping past the input shaft seal, under the dash. The input shaft is what gets pressed when you depress the brake pedal. A bad seal at the input shaft can lead to noises when leaking. Try sticking your head in the footwell as high up as you can (car running and in park w/ e-brake set) See if you can trace it down to that area, I have a hunch you will.