One of the most common ways car problems are noticed is when the vehicle begins producing unusual noises. A common one is a rattling sound underneath the car that is heard when you shift gears or accelerate. These noises are usually a clue that something underneath the vehicle has become loose or damaged and is moving around excessively and making contact with other underbody components.
The exhaust system on most vehicles is connected to the engine, and then runs underneath the entire length of the vehicle, where it ends in a tailpipe at the rear. This system is suspended from the vehicle's chassis or underbody by metal hangers and rubber mounts. These allow the exhaust system to move a very small amount to compensate for vibrations and movement caused by the engine, which could otherwise damage the exhaust system if it were rigidly affixed to the vehicle. If any of these hangars or mounts become loose or damaged, excessive movement of the exhaust system will occur, allowing parts of the system to contact the vehicle's underbody during acceleration or shifting.
Some exhaust systems have metal heat shields that protect the vehicle's underbody components from the heat radiated by hot catalytic converters or mufflers. When these shields become damaged or loose, they can contact the catalytic converter or muffler and cause rattling or banging noises.
U-joints connect the driveshaft to the transmission and drive wheels. These joints allow the driveshaft to move with the suspension as the vehicle travels over uneven surfaces. As u-joints become worn, the clearances in their bearings increase, causing the joints to loosen and allowing the driveshaft to shake and move excessively. This is usually noticed as clunking when the vehicle is put into gear, vibration when the vehicle is accelerating and rattling as the vehicle is driven.