Well you tried... brush-less motor sounds promising, how well was the shaft balanced on the old motor? there is quite some weight on that shaft.
Sometimes they operate totally smooth.
I dont think it was as much of a balance issue as it is the operation of the brush type commutation and the roller bearings used.
Seems to randomly generate a harshness to it that translates to the bit at times.
Well you tried and it did sound a lot better than the day you got it. Hopefully the other new one will work fine.
I hope so.
I think the new motor arrives today.