Vacuum Leak? 1999 Jimmy SLT

Need help to diagnose possible vacuum leak. Experiencing hissing noise and have targeted noise to be coming from under dash. If I turn the control dial from defrost to floor or vent sometimes the leak stops. Air leak also stops eventually but is becoming more prominent over time. Any thoughts?

You need to trace the hoses that go in thru the fire wall, maybe a vacuum operated mode door in the hvac system.