Hissing Sound?

So just recently, as in last evening, I have a noticable hissing sound coming from what sounds like right in front of the dash/firewall - although I can't confirm location.

When I have heard this in other cars it usually means my brake booster is going. Trouble is with this car I have no power brakes and no brake booster.

The car is a 72 Base Dart, so NO power brakes, three on tree (it has a clutch - maybe that's it? seems to change a bit when I put the clutch in, although again not entirely sure). Its a slant six, no A/C.

I would appreciate any help here you can provide.

Mark