Hissing noise from Inlet Manifold on Peugeot 106?

i've just recently fitted a new engine in my Peugeot 106, and after many troubles trying to start it, ended up being flat battery..got it started, and theres some sort of hissing noise coming from the inlet manifold? but everything seems to be in place..:s i've got no idea what it could be? unless its actually the inlet manifold itself? :s idk..any idea's?