If you've been hearing an extra clunk underneath the car, or if it looks like a sparkler at night thanks to a dangling muffler, you might have a broken exhaust hanger. Most exhaust hangers (aka muffler straps) are just thick rubber loops, kind of like really strong rubber bands. Like anything rubber, exhaust hangers eventually wear out, and that's when your muffler starts to suffer.
The best way to figure out if you have a broken exhaust hanger is to take a peek under the car. Follow the exhaust pipe to the muffler. You'll see at least one black rubber exhaust hanger close by. If it's broken, you'll see right away. Be sure to check for other exhaust hangers, as well.