In my 82 300D turbo diesel every time I shut off the engine lately I get a high pitched whistling from the front of the engine compartment. I know I have some vacuum leaks in the central locking and HVAC control unit but when I started looking for them I found that my vacuum pump is functioning properly and holding a vacuum. That was my first thought that the diaphram in the pump had a hole in it.
Could this be something in the AC system since I've only noticed it since I've started using it.

I don't have any performance or MPG problems with the car and the AC is blowing cold(only out of the side and defrost vents). My wife is worrying herself to death that this car is going to leave me beside the road someday but I still have faith in it.