The PCV valve is a controlled vacuum leak, but you shouldn't be able to hear it hiss. If you hear it hiss, that means it's open to the atmosphere (and it is, but not directly). The PCV valve pulls fumes in through the crankcase, and the fumes are displaced by fresh air throuh the breater filter. So while the PCV valve does have a path to open atmosphere, the path is long enough that you shouldn't hear it hiss. If you can hear a hiss from the cab, then that's a sign that there's a direct path between the atmosphere and negative pressure in the intake stream.