High pitched squeal
 

I have a 94 C-220 with digital control for heating and ac. I have recently developed a vacuum leak and a high pitched coming from passenger side footwell area. It only squeals when the system is in the off position. It is very loud at idle and subsides as I accelerate. Has anyone run into this? If I turn on ac or heat it goes away.Other than that the system works fine. Any thoughts would be greatly appreciated. Regards

If you are aware you have a vacuum leak that is causing this problem it should not be too difficult to find the source of the leak and repair it. The reason it subsides when you accelerate is that the vacuum comes from the inlet manifold and is less when the throttle is open.
Geo.

Thanks for the reply ..The vac leak is coming from a valve on top of the plenum on the passenger side I can't see how it is attached but when I wiggle it the squeel subsides there is a green vacuum hose that goes to the top of the valve.I assume this works a door in the plenum? Can you tell me if the valve is replaceable without removing the dash? thanks for any help or insight.Regards