the hiss
I got the hiss as well. I have noticed that this hiss is from the vacuum controlled bypass valve that opens back up upon shutting the car off. When the car is running and being driven, say conservatively, you'll notice the vac/hg (boost) gauge stays more in the -hg area, you actually not using the supercharger as much. Sounds stupid, i know. When you get on it, and see the boost gauge climb, that is when this valve opens. Thus allowing massive flow through the blower, and making our sixers fast.
If ya don't beleive me, open your hood, start your car, and rev it a couple of times while paying attention to the inlet plenum going into the blower. You will notice a valve opening and closing. When i first got my car, this valve got stuck on the connecting hose clamp. No BOOST, freaked me out big time. It was originally intended by ford incase of blower failure. This way, you could still get air into the motor.
In other words, the hiss is the excess pressure being releived from the motor.
If i'm wrong, feel free to correct me, but this is what i've come to know.