I have a Pontiac Firebird 3.8L and I have the same exact problem. I hear a hissing noise when at idle once the car heats up and once I shift into gear, the hissing goes away. I have seen threads suggesting its a bad throttle body, but thats all I can help you out with im not too tech savvy. The noise is a high pitched steady whistle that I have grown accustomed to. Since it goes away I dont worry too much about it.