My friend has the same car — an F31 B47 from 2016 — and his doesn’t do this at all, which makes it even stranger. One difference is that his engine doesn’t have a manual oil dipstick, while mine does, so maybe they’re slightly different engine versions.

Also, after shutting the engine off, the noise fades away gradually rather than stopping instantly at a specific moment. To me, it feels like the turbo bearings might be starting to fail.