Loud exhaust systems can result in tickets in many states. They can also be an annoyance to yourself and others. Your exhaust system could be loud either because of alterations intended to increase the vehicle's performance, or because of failing equipment. Regardless of the reason, there are permanent as well as temporary ways to quiet an excessively loud exhaust system.
Make sure the catalytic converters and the mufflers are working properly. If they become damaged or have been bored out to increase performance, they can be very loud. In most cases, the check engine light will be on if the muffler or catalytic converter are damaged or altered.
Change the mufflers. On most vehicles there are four bolts which hold the muffler onto the tail pipe. If you have dual exhaust, there will be two mufflers. Glass pack mufflers tend to be very loud, but also tend to allow the best airflow. Reactive mufflers are included on most vehicles and are much quieter. Changing your muffler to a quieter one might affect your performance, but can make your vehicle quieter.
Replace your tail pipes. Larger diameter tailpipes tend to be louder than smaller diameter tailpipes. Often larger tailpipes are added to increase performance. If you don't want to reduce your tailpipe size, consider a crossover tailpipe. A crossover lets air travel between two exhaust pipes, which causes the pressure to equalise between the two pipes. The result is higher performance and less volume. If no alterations have been made to your tailpipes, this step will not be appropriate.
Drill two very small holes in your tailpipe, about 10 inches from the end.
Shove a wad of steel wool into the tailpipe. The wad of steel wool should go in as far as the holes that were drilled. The steel wool will work as a damper to absorb some of the sound without a dramatic effect on the vehicles performance.
Thread the wire into one hole, through the steel wool and out the other hole. Twist it around the tail pipe to secure it.
To remove the steel wool from the tailpipe, use a bent coat hanger or some other sort of tool to draw it out. Or, after removing the wire, a good rev of the engine should cause the steel wool to fly out.