Just listened to the video and I have heard people talk about the settling of grease in bearings (clough42 channel where he showed a very nice spindle with PDB made in the US that had a small amount of chatter). So my own view is that it can be a cause but this sounds too loud.

My bet would be the bearings are skipping around as the noise is quite loud. Are the bearings a press or good fit on the spindle shaft and housing? Wonder if they are machined out of spec?

Do you have tools to measure the fits ?

If the bearings spin nicely in your hand try loctiting them place if you are looking to try and repair it yourself.