I thought before that i found the source of my problem with the loose nut at the dash. Sadly i realised that the noise still existed afterwards so the loose bolt was not the culprit.
So after 2 more months of desperate searching i decided to take off the a pillar cover again yesterday. Now suddenly i could hear the noise much louder than before.
Video:
https://1drv.ms/u/s!Aj3rz51JNcMYj4Fb...zN8VA?e=LcvRzb
My conclusion is that the rattle is coming from inside the chassis structure of the A-pillar. I noticed there seems to be a cable running through there, sadly there seems to be no way to reach this cable to fix it down.
So i decided to put some diy sound isolation behind the A-pillar cover to reduce the noise that reaches into the cabin.
It is not perfect but the sound reduce by 80-90% to the point that it is almost unaudible. If i didn't know i would never notice it.
At the next service i will ask the dealer if they know a solution to rectify it completly.