Hissing Sound

Recently I drove from Moses Lake, WA into the Blue Mountains near Pendleton, OR to go camping. This represents an increase in elevation of about 3000 feet. This trip took place in near 100 degree weather.

When I reached the camp site I could hear a hissing sound coming from underneath the car near the rear tires. At first I though it was a leaky tire, then I shut off the idling motor off and the sound went away. After immediately starting the motor again the sound returned. The next morning when I started the motor however the sound was gone.

I have no idea what this hissing noise is a result of. I did crawl under the vehicle and noticed that there are what appear to be plastic vacuum lines that run parallel to the fuel line going from the fuel filler nozzle to the fuel tank. I'm assuming that perhaps the hissing sound was a result of the fuel tank bleeding off some extra pressure because of the increase in elevation?

Any help with this would be great.

thanks,

Automan25
'99 3.8 Intrigue GL

it is probably the gas cap just letting out the extra pressure that it has accumulated from A: the increase in eleveation and B: the heat. it is nothing to worry about as it is doing what it is supposed to