AUIII XR6 VCT 4.0L 200,000km
Recently aquired a strong and constant sucky hissing (i can't think of a better way to describe it) from the engine bay. First thought was intake so checked air box and all the usual suspects, but everything was sweet. To me the sound was coming from the front N/S side of the engine, near the manifold. i thought it may have come loose so checked the bolts and it's hunky dory.
Now I'm perplexed so I put the stethoscope around to try to pinpoint the source but there is nothing unusual coming through that.
The noise does increase (slightly i think) as the revs increase, and when i switch the car off (that happens normally, but) the sucky hissing noise (now that the engine noise is absent) is quite loud and it kind of builds for one last sucky hiss before it is silent 1- 2 seconds after the vehicle was.
Any ideas as to what this could be, before things start getting pulled off...?

Could be a stuffed intake manifold gasket or a vacuum leak. A vacuum hose could be cracked. Does it idle ok? Could also be the booster diaphragm.

I was thinking booster as well. It's the only thing I could think of that retains vac after the engine shuts down.