There is more than one vac line in a car... usually around the throttle body and intake manifold and intake/filter is where you'll find them. One may be disconnected or lose which can cause this sort of hiss. It is literally sucking in ambient air from your engine bay. The thing with vac lines is, 9 times out of 10 they're usless and probably just something extra to keep emissions down or something like that. More than likely you could rip all of them out and not even pull a code. So as far as your car and its well being is concerned, you got nothing to worry about. But as for your sanity, I don't know what to tell you brother. Have you washed your engine bay at all? Anything that could have ripped a vac line loose?