Vacuum leak in HVAC?

[MN12valve]

Hello everyone. Just bought a 1996 2500 5 speed. When I bought it I noticed a valve was placed on the coolant line going to the heater core. The previous owner stated that the cool/warm door in the truck work but the switch may have a vacuum leak so you open the valve for heat. Which does work.

My question is this when the truck is running and when shut down I can hear air leaking behind the hvac switches. Will new switches fix the problem? Or am I looking at something more complicated.

Thinks in advance.

 

[Bob4x4]

You will have to gain access to where the noise is coming from to troubleshoot.Be gentle your dash will be fragile due to age

 

[GAmes]

The blend door is actuated by a cable which is connected to the rotary knob, not vacuum. The directional doors (floor, dash, defrost) actuators are vacuum. The default is defrost, so if that is all you have there is either a vacuum leak or a faulty actuator.