Hi - here's a video of my fan oscillating. It's a 34K miles car. I think the alternator was replaced by PO's mechanic not many miles ago, but some years (it was in collection, not used). I just replaced belt, and spinning the alt shaft, I noticed a slight oscillation of the fan, causing me to believe there is a misalignment in the hub for fan/bearing.
It doesn't appear to be a bent alt shaft as that piece stays true as fan oscillates...
https://vimeo.com/267664671