Sounds like alternator bearing is kaput. Mine sounded exactly like that, than I replaced all drive belt pulleys, new serpentine belt and new alternator and bam! Lovely and quiet.
I don't understand why people use aftermarket belts. The genuine last and last. I have a serpentine belt that has done over 390000 km on a 1GD engine. Still as good as new. Do they cost more ? Not really in the long run. That engine sounds very healthy 😊
aftermark part squeaks go away after a week once they wear in. little bit of water on belt gets rid of the squeak. been through the experience recently with new aftermarket pulleys and belts. Of course genuine is better, can tell the difference in part quality.
This would be even better if it was under load. That cold rattle is much more pronounced. Many thanks as I can hear the same noises from mine at 280k with new injectors. Further confirmation that it was done right under your guidance.