When running the engine without the belt, the sound is not there.
Already tried that a few weeks ago.

Revs were not above 2000 in video 1 and a bit above 2000 in video 2.

Sound is definitely coming from the alternator.

Please take a look at video 3. I think the frequency of the sound is proportional to the rpms

Also, this is an electrical noise. I have installed a Chinese tv module and this module feeded the speakers exactly with that noise so I disconnected the tv module.