If you live in a cold part of the country, and you start the car in the morning after been sitting in the cold all night, you're going to hear a lot of strange noises. Noisy mufflers and other exhaust part don't always mean there are holes in it. Your exhaust is going to make the most noise upon cold starts because your initial hot exhaust gas is mixing with the cold air inside your exhaust system.

After car has been driven, and parked over night, water will condense in the in the muffler. In freezing cold climates the water will freeze, and then slowly melt as the exhaust gas heats it up. Eventually, all that water evaporates and the noise will most likely 'evaporate' too.

Other causes of noises are loose heat shields, weak exhaust hangers, and loose exhaust bolts. As all these things start to rust, they weaken; which causes excessive noise under heavy strain. Your exhaust is under most strain upon cold starts and hard accelereation. You may also notice noises during high speed driving, and coming to short stop. The back pressure that builds up during deacceleration can put a lot of strain on an exhaust.

If you never had your exhaust expected or replaced, you should have it checked for any holes or loose parts. If you're noticing lower gas mileage and generally more noise while you're driving at a constant speed, you may have a hole or other leak. Good luck!