N400 Exhaust Flap Issue

Sounds like your exhaust flap motor might be on its way out. The symptoms you're describing are pretty typical. Has the car been running any differently besides the noise? Any noticeable drop in performance or fuel economy? Also, have you tried visually inspecting the flap and its connection to the motor? Sometimes it's just a loose connection or a bit of corrosion causing the issue.

Thanks for the quick reply! I haven't noticed a huge drop in performance, maybe a little sluggishness at lower RPMs. Fuel economy seems about the same. I haven't actually looked at the flap motor itself – where exactly is it located and what should I be looking for? Is it something I can potentially fix myself, or am I better off taking it to a workshop?

The exhaust flap motor is usually located near the exhaust, often close to the rear axle. You'll be looking for a small electric motor connected to the exhaust flap via a linkage. Check the wiring for any obvious damage or corrosion. While it might be a simple fix like cleaning a connection, a defective flap motor is a common issue on these. Considering the error code and your symptoms, the flap is likely defective. Unless you're comfortable working on exhaust systems, I'd recommend a workshop. They can properly diagnose it and replace the motor if needed.

Okay, that's what I was afraid of! Thanks so much for the detailed info. I'll take it to my local workshop. They confirmed it was the exhaust flap and replaced it for 190€. Runs like a charm again!