beyond the potential broken broken hose.. the crankcase vent valve (part #1 in your link versace). These things are notorious for making funny noises including the hissing sound you hear. The hissing sound is coming from pressure built up inside the crankcase. Have the vent valve replaced and your noise should go away. The other thing you might want to check is your air filter. Make sure the air box is sealed properly and that the air filter is seated properly. Any sound of pressure bleeding off from the engine will be amplified if the air filter isn't seated or the box isn't completely closed. Ironically, I think there is a thread about this just recently.
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