There is a bypass line built into the inlet manifold on the inside that runs to the throttle body. If your throttle body gasket doesnt have the right holes punched into it you block the bypass off and apply full manifold vacumm to the engine.
When this happens you are applying negative pressure the entire engine, hence why the noise isnt there with the hose disconnected. The noise you here is air being sucked INTO the engine either thru oil pump, oil filter location or anywhere like that.
What manifold does you have, bigport rwd, fwd converted??
Your sound isnt a moving part, the release/normalization of pressure when engine turned of is the fade out noise.
This is what happens to me and hundreds off others that went to a rwd converted fwd manifold.
Easiest thing to do (if you have a fwd>rwd manifold) is run a t piece off camcover to manifold line and attach it somewhere before the throttle body. You may need to fiddle with idle after doing this.
If you have a rwd manifold then either do the same as fwd>rwd or pull your throttle off and punch all the holes into the gasket. clean your throttle body while your there.
