Engines are massive pieces of machinery that tend to be pretty loud when they're running, which is why mufflers are installed on cars and trucks to keep sounds at a more polite, tolerable level. Gearheads and auto enthusiasts may specially design their cars to run without mufflers so their presence is well-known, but many states have noise pollution laws in place that require vehicles to be equipped with mufflers, so it is important to take care of any problems that may arise.
A muffler consists of a bunch of tubes that can cancel out the sounds an an engine makes, and it is affixed at the tail end of the exhaust pipe. They are usually made of metal, so they are susceptible to rusting and corrosion. When a muffler is corroded, a hole can form, which can upset the balance of the muffler and the car will be noticeably louder. This can be a problem, especially for those states in which mufflers are required by law, but at least it is easy to diagnose. If you notice that your car is louder than usual, there is likely a problem with the muffler.
In order to confirm that this is the issue, you will need to jack up the rear of your vehicle and start the engine. Covering the end of the exhaust pipe to create pressure will force the gases to find another escape route, through the leak. You can get underneath the car to listen for where the sound comes from, and you can also use your hand to feel for a stream of air.
When you find a leak, this means that it is time to replace the muffler. It may be tempting to find a way to plug up the hole, but once the muffler begins to show signs of wear, it will just continue to rust or corrode, so replacement is the best option. This will not only keep your ride quiet, but it will also help you avoid the hefty fines that you can get if your car is too loud on the road.