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.
I'm not the first to discover this by any means. There are a variety of posts discussing this issue, this one from DuntLed_Racing is especially insightful:
Per the manual the simplest way to do this process would be to drop the rear exhaust hangers (shown in video) and pop out the other hangers as needed to get clearance. Having the exhaust off completely makes for a nicer video though.
The new part number is: 68319331AA
This site was suggested by DuntLed_Racing where they sell them in pairs:
https://mfer-1.com/shop/ols/products/new-style-active-exhaust-springs
I purchased mine on eBay here: