Insulate, insulate, then insulate some more... That's the best advice...I've got A/C so I added some thick jute carpet padding on the floor,roof, jute padding under the back seat and wheel well areas,on the package tray and rear divider as well as foil type behind the door panels...And I inadvertently quieted the whole seating area down dramatically... I've got Hooker 1 7/8 super comp headers and a complete TTI 3 inch "H" pipe{which is louder than the X}with the super turbos and there's no drone or resonance...You have to roll the windows down if you want to hear it loud...And it definitely sounds different to the people outside or behind the car on the road...