Have a 2006 Ford Freestar Limited with multi-zone climate control. Have noticed a significant hissing as the vehicle is warming up to operating temp. Upon investigating, I found that the hissing originates from the in-car temp sensor while the fan speed is high too warm the vehicle. I found the issue by covering the in-car temp sensor grille on the panel. No hissing! Popped the panel, covered the sensor housing attached to the hose - No hissing!. Obviously the sensor needs the airflow to maintain the inside temp. The hissing reduces once the van is warmed up and just maintaining in-car comfort. The system is operating normally, otherwise. Can anyone tell me what's going on and can I remedy the situation/ Any help would be appreciated!