Gas Tank Question

When I take the cap off my tank it sounds like I'm opening a 2 liter of cola. Its a dry hissing sound, do you all have any idea why it happens?

Your supposed to have a hissing sound when you remove the gas cap. That means it is holding a vacuum like its supposed to. That vacuum is what keeps gasoline vapors in the tank, so if the tank leaks a little vapor, it will leak in not out; however, if the tank does not hold a good vacuum (leaks too much), then the vapors will leak out and that is NOT good. In short, hissing sound when removing gas cap, good vacuum in tank.

hmmmmm...
mine DOSEN'T hiss...

is this a huge problem?