Engine vacuum is a relative thing.... IIRC, you are in Calgary which is about 3500 feet. This makes 15 in/Hg at idle pretty good.... (you are also going to be interested in anything that the gauge is doing - pulsing, nosing over, anything...). Those closer to sea level will see idle vacuum closer to 18~20 inches. 30 inches Hg is considered "perfect" vacuum.... ain't gonna happen in the intake manifold...
Even though this car is computer controlled , the basics still apply.... "suck - squeeze - bang - blow"... vacuum gauges, compression testers and the whole nine yards are still very important tools... If a sensor reading isn't outside the "window", there probably wont be a code but there could still be a problem....
Even though this has been an on going concern and you have described it a few times, I'm still having trouble correlating "exhales".... this is one of those things I wish I could here at least once....
I am assuming that you tried locating the noise with a piece of hose and it didn't help? Let's grasp at a few straws, then.... try letting the car idle.... with it running, remove the oil filler cap. Does it come off normally? With the oil filler cap off, turn the car off - does it still "exhale".
Just to refresh ones memory - the sound is definitely underhood? Other than this noise, no other symptoms exist?
Hmmm... a top posted over HTML reply.... Ooooh, I are bad.
<DIV>Does anyone know the vaccum on a stock 1993 5.0/Cobra? </DIV>
<DIV>I'm getting a consistent 15, which I assume is good, but there's
definitely a leak as the car "exhales" when I turn it off. I can't find the
leak for the life of me, but it has to be an accessory.</DIV>
<DIV>How do I reconcile 15 with the exhale?</DIV>