Intermittent hissing sound

Hi. I have a 2003 Trailblazer LT EXT. I have an intermittent hissing/squealing sound while AC is on. Usually hear it at startup when AC is on. Also, hear it when I turn AC off. Sometimes noise is soft; other times loud. Will make noise for a couple of seconds, then stop. Then start again when acclearating up to 45-50 MPH. Don't hear it as much above 55 MPH. Can hear it intermittently some days, constantly other days, and some days not at all. If AC stays off, I don't hear noise at all. I have taken it in to dealership 3 times. First, they couldn't hear it so couldn't fix what they didn't hear. Second time, they changed belts; didn't work. Third time, they changed AC actuator; didn't work. I'm taking it in again on Monday. Any suggestions?? Solutions??? Thanks.