What causes the front-end noise when driving a Mercedes W211?

[MacCourtney]

There are several reasons for the front-end noise when driving a Mercedes W211: Uneven engine cooling: After driving for a while, the engine temperature becomes very high. When the engine is turned off, the temperature of various components decreases, and thermal expansion and contraction cannot occur uniformly. This inevitably leads to gaps between internal metal parts due to cooling contraction, resulting in friction and noise. Exhaust pipe cooling noise: Typically, the exhaust pipe temperature is around 200 degrees Celsius during cold idle. When the car is under high load or there is combustion of mixed gases in the exhaust pipe, the temperature can be even higher, causing the exhaust pipe to expand and contract thermally, producing noise. Catalytic converter cooling noise: The normal operating temperature of the catalytic converter is between 400-800 degrees Celsius. After the engine is turned off, its operation gradually stops, and the temperature decreases, causing thermal expansion and contraction of its internal metal components, which then produces noise.