hissing noise
 

I have a 1985 cd. When transmission shifts I get a hissing noise which goes away when I accelerate or back off . Seems to be at part throttle only.
Anybody had this problem?:confused:

Air leak associated with the turbo.

Where does the leak sound like it is coming from?

Since the Vacuum Valve starts off with a lot of Vacuum and at full throttle has almost no Vacuum (hissing would stop) you symptom might also be a Vacuum Leak.

As Kerry said some areas that can leak due to the Turbo Pressure.
So besides the A leak in the Intake Manifold Gasket or may be the hoses going from the Manifold to that Over Pressure Switch and on to the IP (ALDA?) could be leaking. (the hose down by the Waste Gate.)

To check they Intake Manifold Gaskets with the Engine cold spray Soapy Water (dish soap that makes a lot of bubbles mixed with water) in the area where the Manifold meets the Head and any other areas you think would have a pressure leak. If there is a leak it shoudl bubble when the Engine is running and reved up enought for the Turbo to pressureize the Manifold.