under my dash, in my truck, when the brakes (power disc) are applied. Brake booster leaking? Still good brakes though.
That's my guess.

My 74 truck used to hiss while braking. I replaced the pwr booster line and it went away.

I would check your booster and lines carefully. The incident that drove me to parking my car and begin tearing it apart, was when I burned 3 exhaust valves.

My engine started to run "off" and was down on power. I did a compression check and had 3 dead cylinders, no compression.

What had happened, was that my brake booster had developed a leak. the leak was so bad, it made that half of the intake manifold go lean, and burned 3 of the valves on that half of the intake plenum...the 4th was showing signs of burning lean/hot and was about to burn as well.

I knew about the leak, but thought "well, my brake pedal is still good, and everything feels ok, so it must not be that big of a deal so I'll just keep driving it like it is"