Many B9 S4s are experiencing an annoying exhaust rattle related to the active exhaust system. Some S4s, like mine, even throw fault codes including P0477, P0475, and P049F showing there is a problem with the valves. This is often caused by a weak or broken spring inside the exhaust door control unit. Interestingly Dodge Charger and Challenger models have the same Küster brand actuators flaps/valves inside their exhaust too. Facing the same issues a newer spring has been made available by Mopar that eliminates the rattle.