Look for a vacuum leak. It will make a hissing sound while idling. Sounds like your problem.
April, 22, 2012 AT 3:33 AM
Just to add to this one if the ac works turn it on see if the idle smooths out?If it does you need to have the ecm updated with a new calibration file that raises the idle up to where it should be. There is a bulletin for this issue have ran into it many times myself.