If you clued us in on what you're driving...
What year?
Transmission?
Seriously. We need to know what you're driving because these 986s have technical/parts specifications that are all over the map. Maybe include that information in your signature.
A hissing shuld be easy to diagnose, and absent all that ancllary information you provided. Get one of the stethaascopes devices and start poking around with it. In the alternative... remove easy to access hoses one at a time and listen for the hissing to stop. You might find the problem. Good luck.