Vacuum Bleed off

I have a 1996 Windstar 3.8L, when I turn it off there is a sound almost like a dying cat. It only lasts for about 5-10 seconds. The van runs fine. I was talking to a friend who said that he thought that it could be vacuum bleeding off. All I know is that it is very annoying and the Wife is not happy. I can not seem to find out what it is.:banghead:

There are a couple of things that this could be.....vaccum bleeding off is 1.....but there are a number of possible places that this leak could be.

Another thing is that it could be the A/C system.
To see if this is the case......
Try setting the air control to the VENT position a few MINUTES before you shut off the engine.