Hissing brakes help please

[dave`12]

Not sure where to put this. My daughters 2002 S10:

when you push the brakes light (but enough to stop truck) all is normal.
brakes hiss when you push medium hard. the hissing noise is in the cabin, the plunger going into the power booster. Brakes still work, and feel like power brakes.

Never experienced this. Could it just be that something suppressing the noise has deteriorated?

Hoping that I don't have to replace the brake booster. It's quite tight there. If I do, could someone point me to instructions? I only see 2 bolts on the booster to the master cylinder, nothing else holding all this together. Not sure if removing them will allow me to pull the master cylinder away and pull out the booster?

Any advice appreciated!

Dave