Does it have 2 exhaust pipes in the back? Is there a badge that says GT or a coiled snake on the side of the car? If yes, then that means your car has a V8 engine, and it's supposed to be loud. You may have noticed the car accellerates really quickly when you hit the gas. No, it's not broken, it's supposed to do that!
If you want a silent car, buy a hybrid or electric car. Mustangs make noise; most buyers think that's a good thing.