I just want you all to do a simple task to me while you're driving in your stang. The task is to drive normally but when you get to 30-40 mph, shift to 5th gear and press on the gas pedal and see if there's hissing/air sound present (with windows up). I'm asking you all this because I want to make sure if that's it's normal. The sound seems to be coming from the very back of the car. Peharps it could be exhaust leak.. but please test it to confirm if it's normal or not.. thanks