If you hear a loud rattling noise when you put your car in reverse neutral or drive at low speeds, there are a number of different issues that could be causing the sound. Most likely, the noise is due to a loose component knocking against other parts of the car, but it could also point to other issues under the hood.
First things first - figure out where the sound is coming from. You can do this by starting the engine and leaving the transmission in park while the motor idles. Walk around the car to see if you can figure out where the sound is coming from. You may need to pop the hood to more easily locate the source. If the sound is coming from the engine area, you may be dealing with loose or broken engine mounts or brackets holding the exhaust pipe or other parts in place. To check these, you will likely need to jack up the car and manually inspect the mounts for damage.
When the noise seems to be emanating from the rear end of the vehicle, the catalytic converter might be to blame. Inside the converter, there is a honeycomb-like ceramic filter coated with catalysts that neutralize harmful toxins as they pass through the exhaust system to reduce emissions. This porous plate can break, and the shards will rattle around in the converter, which may lead to a rattling noise when your car is idling or traveling at low speeds.
Another possible cause is that the timing chain is loose. Not all cars have timing chains, but your owner's manual should contain this information if you're not sure about your particular make and model. A loose chain will not just cause rattling, but you may also hear ticking or slapping noises. If your vehicle has a timing chain and you suspect this is the source of the sound, you can test your theory by revving the engine. The sounds will change as the RPMs rise and fall.
There are a number of other issues that could be causing the sound, such as a cracked flyplate in a car with automatic transmission or a clutch going bad. A failing clutch might result in part of the system becoming disconnected. It will then bounce around and create a racket inside the transmission housing.
If the sound comes from the wheels, you might be dealing with loose brake calipers or brake pads, which will result in more of a clicking sound than a rattle. While you're in the area, you can also inspect the tie rods that connect the car's steering system to the wheels. They have ball joints at one end, which can cause a rattling noise when they wear out.
In fact, the ball joints in the suspension system could be the cause of the issue as well. The suspension ball joints are designed to withstand a lot of pressure from the car's constant bouncing, and the metal-on-metal part requires lots of grease to move freely. When the ball joints wear out, the result will be a loud rattling or knocking sound, which will be most noticeable when driving over bumps or dips in the road.
Other parts, such as control arm bushings and frame bushings, may cause loud noises if they wear out or are damaged. Bushings are rubber pieces designed to prevent metal parts from rubbing against one another. You can also check to see if the rattling only occurs when the air conditioner is on. There is a clutch that engages the system when you turn the air conditioner on, and if it fails, you may hear a knocking sound.