2002 Suburban odd noise

My Suburban has started making an odd noise. It sounds like a air pressure release valve, kind of a fffffffft sound. Hard to describe but it sounds like air escaping. It seems to be coming from under the dash but it is hard to pin point. Sometimes it does it pretty loud and long when I first start the engine. It does it about every minute while driving. I let it run in the driveway and never heard it. Anybody have any ideas? It is a 3/4 ton with all the bells and whistles.

My Suburban has started making an odd noise. It sounds like a air pressure release valve, kind of a fffffffft sound. Hard to describe but it sounds like air escaping. It seems to be coming from under the dash but it is hard to pin point. Sometimes it does it pretty loud and long when I first start the engine. It does it about every minute while driving. I let it run in the driveway and never heard it. Anybody have any ideas? It is a 3/4 ton with all the bells and whistles.

put the a/c heat in the off position and see if it makes noise..
could be a damper/hvac control vac leak ..


on the 2002 this should be controlled by motors though..