Hissing noise coming from behind under the intake manifold??

Hello, sounds like a possible vacuum leak. Testing should be performed to pin point the location of the leak, many technicians would "richen" the area around where you hear the hiss noise and see if the engine running improves. This is done with a gas like propane introduced into the area. I would not recommend any of this for the "backyard" mechanic, but would suggest seeking some professional assistance for what is likely an easy diagnostic test for someone with the proper equipment taking the appropriate safety precautions.

Hissing noise when key is turned to the ON POSITION is likely fuel pressurization inside the fuel rail. Professional *hands-on* testing which includes fuel pressure test as well as full OBD1 scan test is recommended..