Agree with coralman and if your fuel tank pressure system is working correctly you shouldn't hear any sound in removing the cap. The emission system line going to the top of the tank removes the fuel vapors, filters them though the charcoal canister behind the far left front bumper to remove the impurities, then they're pulled into the intake by vacuum to be burned through the engine.
This same line acts as a breather so when the pump runs it doesn't pull all the fuel/air out of the tank which can cause the tanks to collapse. Sames true when the gas expands when hot out and the fumes are vented through the emission system to relieve pressure. While the correct tank caps have a vent/vacuum pressure setting I believe these are only used when pressure/vacuum is extreme and the emission system is not working correctly.
The whole idea is to vent the gas fumes through the engine with an emission system rather than venting them through the tank cap vent so they aren't released into the air/atmosphere.