If your vehicle is showing loud hissing sound when unscrewing the gas cap, here are the most likely causes and the parts you'll need to fix each one.
The vent valve allows fresh air into the EVAP system to relieve pressure and vacuum. If it gets clogged with dirt or fails in the closed position, excessive pressure or vacuum builds up in the gas tank.
The purge valve routes fuel vapors into the engine to be burned. If it gets stuck open, the engine's vacuum constantly pulls on the fuel tank, creating a massive vacuum that hisses loudly when the cap is opened.
The charcoal canister filters fuel vapors. If you frequently 'top off' your gas tank after the pump clicks off, liquid fuel can saturate and ruin the charcoal, physically blocking the system's ability to vent.
Many gas caps have a built-in mechanical relief valve to prevent extreme pressure or vacuum as a failsafe. If this valve sticks, the tank won't vent properly until you unscrew the cap.
If the fuel tank was recently removed (for example, to replace the fuel pump), the vapor lines connecting the tank to the EVAP canister may have been pinched or crushed between the tank and the body during reinstallation.