anyone know what this noise is?

[USDM240]

about a week ago after i shut down my car, i noticed a hissing sound coming from the engine bay. i cant seem to pin-point it, but it sounds like its coming from near the rear of the head. sounds kinda like a turbo spooling down, but its not since i am turboless. anyone know what this might be. it sounds like it might be a blown head gasket to me, but just wanted to get some other opinions.

[Thee 240sx Owner]

It can be anything from a vacum hose leak to a gasket leak in the intake madifold... a blown head gasket will be indicated by a milky color in the oil...

[USDM240]

well, i know its not the headgasket, but i still cant figure it out. still the hissing noise and no answer as to where its coming from. ?????

im stumped....

[250sx]

freeze plug?

[USDM240]

didnt think about that. ill have to check. could it possibly a vacuum leak?

[p00t]

sounds like a vacuum leak. check the brake booster hose where the check valve is. follow the hose from the booster right behind the engine theres a brace/bracket where it holds to the firewall. the check valve is there, mine has popped off there.

if not then you gotta check all lines for cracks etc