Fumes Coming in the AC Vents
 

I have a 2008 Mazda3 GT. Whenever I run the fan through the air vents on either the ac or just outside air, it sucks in a strange smell like gas fumes. The dealer looked it over and said nothing is disconnected or leaking. So does anyone have any idea what it can be? I live in Florida so we deal with extreme heat but no corrosion issues. I can't stand the smell. Thanks.

Additional information on this dilemma....these fumes that come in the car are at there worse when I am at a stop light or stopped in traffic with the fresh air button on.
Thanks for your help.