Yep, this happens on all flavors of cars and trucks that have A/C. Basically moisture forms inside the ducts when the A/C is turned off, and starts to smell. When you run the A/C, it dries the air, and therefore the ducts, --- smell goes away. Turn off the air, warmer moister air passes through and condenses inside the ducts again. It can be irritating.
07 GT Convertible. C&L Intake, SCTx2 91 Torque Tune by Bamachips, Ford Racing X-Pipe, Pypes Mid-Muffler System, Webelectric DRL's & Sequential Turn Signals, Eleanor Grille.