Ok guys I finally took a look and I have to say thank you for all your input. I think this is a really stupid one but thank god nothing happened:
I followed your advice and removed all the belts, turned on the motor and no weird noise. Everything sounded ok. So I decided to first put back the alternator belt. While I was moving the alternator, I thought it was kind of weird that it was wiggling a little bit so I thought I'd check the lower bolt that holds the alternator in place. Well it turns out, that there is no bolt anymore. I guess that the alternator moving is causing the noise.
Here's what I think happened:
I recently had to replace my alternator. When putting the new one in, I suspect I had the lower bolt loose while tightening the belt via the alternator upper bolt. And I think I never tightened the lower bolt after I was done with the upper bolt. I think I remember during my drive home after my last trip, I heard a weird sound that sounded like something falling out on the bottom or something like that. I'm pretty sure it's this bolt that came loose and fell out.
How can I identify the bolt that I need without having to remove the alternator. I would really like to avoid taking it out again as this was quite a pain.
Thank you so much!!!
Nico