Lately I have been hearing an abnormally bothersome noise that appears to originate by my left foot away from the brake pedal. The sound appears to be an air stream escaping or gasping (like a sucking vacuum). The sound stops when I apply pressure to the brake pedal. It has become more annoying and concerning that it could significantly effect the performance of my braking. Please help. Thanks.

My 2003 V70 has been doing the same thing for some time now. It seems to be temperature dependent - worse in cold weather. In my case it's a low volume, high pitch whistle. Tapping the brake pedal the tiniest bit quiets it. Not enough of a leak to affect braking power. I guess the only cure is to replace the booster, but I'm procrastinating.

You could have a vacuum booster that is failing, though the sound usually starts when brake pressure is applied. Have you looked under the dash while actuating the brake pedal by hand?

Honestly, the sound is a constant hissing sound and stops when the brake pedal is applied. You refer to a "booster"....do you mean a "Booster Vacuum Switch" or a different specific part? Where under the hood should I look and what in particular should I be looking for? Please explain. Your insight is greatly appreciated.

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.