exhaust hissing

brado

i know its normal for our cars to have hissing sound under deceleration with an aftermarket catback but i think i may have problem. a couple of my friends thought i had an exhaust leak but i told them all 370s have the hissing sound. the car doesnt have any ticking noises or anything like that, its just when u hit the gas it hisses as the rpm's go down. one of my friends is a mechanic and he put it on the lift this morning. when he hit the gas the hiss is louder on the passenger side and u can feel air coming out of the flex joint (only when u hit the gas though not at idol). is it an exhaust leak? should i worry about it?