Chrysler Repair: Sound from New Yorker Dash board, automatic temperature control, chrysler new yorker

Question
I have a Chrysler New Yorker Fifth Ave with  a V8 Engine and I think the year is either 85-86, I'm not sure right now... :^)  Anyway the problem I'm having is I have a noise comming from my daskboard from behind the environmental controls.  It's a high pitched sounds and it only goes away when I press on the gas or just start the car cold.  Any Ideas? Thanks!

Answer
Hello James,
What you are no doubt hearing is the air suction produced by the vacuum created by the intake manifold of the engine. When you step on the gas the throttle valve opens and so the vacuum decreases and the sound disappears until you once again let up on the gas. That vacuum source is used to operate the various doors that direct the air associated with the heater/a.c. unit to various openings. The vacuum comes to the unit via a thin plastic line from the engine compartment, which usually branches off of the main vacuum hose that goes to the power brake unit. That thin line goes thru the firewall about midline of the body, right in line with the heater/a.c. unit, where it connects to a part of the unit called the power-vacuum module (this assumes you have the automatic temperature control system becaue you speak of this as "environmental controls".) Even if it is not such a digital unit, there will still be a vacuum operated door system in the conventional heater/a.c. unit.
I would suggest that you look under and to the right of the unit (near the floor to the right of the hump) for a vacuum line (probably wrapped together with some other wires as a 'harness') as I have described coming to the unit thru the fire wall. It has a small check valve in its tube. I suspect that the line has become partially disconnected at the valve, or at the p-v module itself, or a crack has develop in the line which means it should be replaced between the engine compartment and the cabin. This should be a pretty simple job for an amateur or an inexpensive job for a mechanic to complete. It is also possible that one of the vacuum lines between the p-v module and the door actuators has cracked or become dislodged internally to the unit. That too should be fairly easy to fix, but try to look at the main incoming line first.
Roland