Hissing sound

This would be the catalytic converter is plugged. Need to get to an exhaust shop to get it replaced.

Thanks Rowefast

Your welcome. If you want to verify, just get it warmed up and park in a dark area, leave the engine run while you look at the cat. It would be glowing orange, which means it is plugged.

Mine was a leak in the heater core. When I punched the gas I would hear high pressure air going threw the leak making a hissing sound. Also when the truck was idling it would make a ticking or knocking sound