I'm pretty sure it is known that the cause of the milling noise is a worn bearing. I have always thought that regen was the culprit and I'm thinking Tesla knows this too which is why I have speculated that Tesla keeps "lightening" the regen in order to minimize the forces on the bearings during regen. I bought a BMW i3 as our second car a few weeks ago and I noticed that the regen disables when going over bumps in the road. I wonder if BMW engineers realized in advance that regen over bumps is bad for the bearings so they put this protective measure in place. If this is true, perhaps Tesla can do the same and bring back full regen again!