Hissssing noises r a big clue
use a can of starter spay(carb cleaner works too) and MIST, dont hose down, the area u wish to test. for example if u want to check for craks in a vaccume hose, lightly spray the hose w/ur car running in idle. when u spray, step back and listen to the engine/RPM'a for a change/rise in sound. if the rpm's rise, u found ur problem. if no change, then keep searching.
the hose connections where the hose clamps begin to crack up sometimes, so there can be hard to find cracks after a while.