If your vehicle is showing rattling noise underneath the car like marbles in a can, here are the most likely causes and the parts you'll need to fix each one.
The thin metal heat shields around the exhaust system frequently rust at their mounting points or get bent during undercarriage work. This causes the metal to vibrate rapidly against the exhaust pipe at specific engine RPMs.
The internal ceramic honeycomb structure of the catalytic converter can break apart due to age, impact, or engine misfires. The broken pieces bounce around inside the metal shell, creating a classic 'marbles in a can' sound.
Rubber exhaust hangers can tear, stretch, or fall off, allowing the exhaust pipes to sag. This lets the metal exhaust components rattle against the vehicle's underbody, crossmembers, or suspension.
Internal metal plates and tubes inside the muffler or resonator can rust and break loose over time. These loose metal pieces will rattle inside the casing as exhaust gases flow past them.
Often described as marbles rattling under the floorboards or hood, this occurs when the air-fuel mixture ignites prematurely in the cylinders. It usually happens under heavy acceleration or when using low-octane fuel.