When ever I shut off the car I hear this loud hiss like the sound of letting air out of the tires. It's a 94 GT bone stock with a catback exhaust. It idles a little rough and seems to be lacking down low. I've tried to "propane" all around the intake manifold and also tried carb cleaner and electronic cleaner around the intake manifold gasket but I didn't notice any change in how the engine idled/speed up. It sounds to be coming from between the engine and the firewall.