4onthefloor could be correct. A vacuum as we know is caused by removal of air so when you open the cap ambient air pressure rushes in to fill the vacuum and may cause the whooshing sound you hear.The incoming air also pushes the fumes out. Or you're right and the current summer heat is building pressure in the tank that releases to the air. In any case I wish I could name a good source for a "correct" fuel cap.Maybe you should by any aftermarket one that fits until you locate the one you really want.
