Engine brakes are systems in large trucks that use the engine's power to slow down the vehicle. These systems are more effective at slowing heavy vehicles than traditional friction brakes, but they can be very loud. Many municipalities ban engine braking near residential areas.
One common form of engine braking is the jake brake, a system that opens the compression valves inside the engine. This lets out the compressed gas in the cylinders and slows the vehicle substantially. When the valves open, the gas rushes out and makes a distinctive and very loud chattering noise, which can be heard for quite some distance from the vehicle.