ilden bad on 1999 Mazda 626

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

Asked by for the 1999 Mazda 626
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.