Truck saying "Tighting the Fuel Cap"
 

I just read all the post concerning the fuel problems. I have a '04 Chevy Avalanche and I'm getting a signal saying "tighting the fuel cap". I brought a fuel cap and it still saying the same thing. Now, when i take the cap off a "hissing sound" come from the gas tank, sound like it's releasing some pressure, then stops after about 1 minutes. Can somebody give me some insight on what this problem may be and how it can be fixed without going to the "stealership".:)