The "hiss" or "woosh" sound is normal. Your fuel system is pressurized.
Your gas cap gasket is worn out though so when the system is pressurized, fuel can leak past the gasket.
Would think this wouldn't manifest itself except when the take is full (or overfilled)...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car