Cjm99sierra

Hey guys I need help. So I'm trying to diagnose where this noise is coming from under the dash on passenger side on my 99 sierra 1500. I know it's not the blower motor as I removed it and the noise persisted. It sounds like a squeak or a creaking noise that only happens when vehicle is in on position (engine running or not) and its especially noticable with blower fan is off. You cant hear it under the hood but it's the loudest at the vents for the windshield. I'm lost please help. If you need anymore info please let me know.