ilden bad on 1999 Mazda 626

ok so were on the intake manifold and does that causes it to hiss?

1 answer
The hissing sound is most likely a vacuum leak, was all the old gasket removed prior to refitting the new gasket? Again try pouring a little engine oi where you hear the hissing sound with the engine running, if the oil gets sucked into the engine and the engine briefly runs smooth you have a leak from the gasket..
With the engine running you may hear a slight "hiss" which is fuel running in the fuel rail.