Ok, think I found the issue. I marked it with a red arrow on the pic below.
If you remove the door rubber trim around that area, you can slightly pull down the head lining (dont pull too hard, as it is locked in place by the airbag cover). There is a cable-channel wrapped in some kind of fabric that sits in a plastic U-formed cable clip.
What I did was to just move around the cable-channel a bit and now the annoying sound is only audible if you have really good ears
If I find time, I am gonna wrap the fabric of the cable-channel in some other insulating material. My guess is, that the cable-channel is touching the plastic cable clip which results in that annoying sound.
If I remind myself during that fix, I may take some pictures to better explain the issue. However it should be a 5-min fix as its not really much work.
1. Open the driver's side door and remove the upper part of the rubber trim like on the picture below:
2. If you now pull the fabric of the headlining down just a tiny bit (don't pull too hard as it is hold in place with the airbag cover) you will see a white fabric, that is used to organize a few cables as well as a black clamp that is used to secure it. See below picture for clarification:
3. Get a piece of rubber as shown below:
4. Place it between the clamp and the door trim/headlining like on this picture below.