These Coleman 400's are LOUD, so loud I wear earplugs when cruising. With that in mind,I added a secondary Kolpin silencer to the stock muffler. It helped a bit but the thing that has helped the most is foam sound deadener. I added the foam deadener to the inside of the plastic front seat base,covering the whole surface. I also added a piece on the driver side plastic cover and the side of the gas tank facing the motor. I used standard acoustic foam known as eggcrate and contact cemented it in place. Very noticeable reduction in the noise from the engine and drivetrain. Much more enjoyable ride for just a few bucks.