There is a plug on top of the fuel pressure regulator. You should have no more than 4.5 psi there. If there is more than that it will need a regulator or a regulator kit. Too much pressure and the injector will not be able to open. There is also a relief valve In the injector holder that could be opening if there is to much pressure. That could be why the hiss from the air box. if pressure is ok then look at injector replacement.