A heat shield is a stamped metal panel (usually aluminum or steel) placed around high-temperature exhaust components like the catalytic converter, manifold, or muffler. It reflects and dissipates radiant heat to prevent it from damaging nearby heat-sensitive parts like the fuel tank, plastic bumpers, or the floor of the cabin. It also prevents dry grass from catching fire if the car is parked off-road. Loose heat shields are the most common cause of metallic rattling noises coming from underneath the car.