This has been happening for a couple of months now, It doesn't happen every time. Surprisingly not on cold starts. But now it's a more frequent noise. I don't know what it could possibly be. The car drives as usual and there's no noise or any sort of rumbling or shaking. Only when the car starts.

Here's a video,

Thank you!

- 2012 e350 4matic - 115k miles

 

To me it sounds as if the starter have difficulties disengaging.