[jlmjr]

My 96 gt is making a hissing noise under the hood. I've had it checked by 2 mechanics. They definitely hear it but found no vacuum leaks. It sounds like a hiss, but it also kinda sounds like a spinning noise b/c I can kinda hear it spin down when I shut the car off. Does anyone have a idea what this is or if it's common?

[GTRaptor]

Could be noise from the idler pulley or alt bearing.

Use a screwdriver as a sthetoscope, put the end against the suspect location and the handle against your ear.

[housermj]

i have a 1997 gt and it make the same noise. its like a hiss but loud hissing. and when you cut the car off its like it hiss for a sec. like air moving to me it sounds like it is coming from in side the throttle body .i took off my cold air intake at the throttle body and if i put my finger on a little hole just inside the throttle body it stop but runs bad. tell me if you find out what it is.