Model 2001 -Suddenly loss speed and have a "Hiss" sound

I was driving my Truck @ 45mph, suddenly it loss speed and make a "hiss" sound. I stopped the engine for 10 minutes and run ok again, but 5 minutes after that, the "hiss" sound comes back and loss power. Any idea what can be wrong?

akyles@hotmail.com

Check your vacuum lines for a leak.

Sounds like one fell off its' connection.