Ok I took apart the metal case around the power supply. Cleaned the fan and checked if anything was wrong with it. Couldn't find anything special. Maybe it's the ball bearings, but I can't replace/clean those because it's a cheap plastic mold fan.
Was pretty fun by the way to take that thing apart.
Edit: haha! I fixed it by putting a small piece of toilet paper between 2 vibrating parts.
Edit2: Well, it seems the noise isn't completely gone yet, but at least it's much better now.