Motor Milling Sound
Earlier Tesla vehicles, 2015 and back, will eventually begin to have a milling sound or maybe a whine or grinding noise.
The reason is a design problem -- the inductive motor is eating up the bearings.
Power provided to the stator creates a moving magnetic field, which causes the rotor to turn. But residual current induced in that rotor needs someplace to go... and so it goes through the bearings as they are a conductive path back to ground. Over time all that electrical energy constantly going through those metal bearings, which were never designed to carry current, eventually start to pit the balls and bearing races... and so the bearings fail. This is not your fault.
Electromagnetic energy needs someplace to go, so it needs a discharge path. There is a discharge charge path in the motor, but it is pretty inadequate in these earlier cars with drive units lower than version Q. One solution would be a brush on each end of the motor shaft.
But for those who are experiencing this problem, the good news is there are only two bearings involved. It is quite an ordeal to drop the motor substructure, crack open the drive, punch out the bearings, etc. But at least the problem is very clear and simple.
I've heard that there is a 'ceramic bearing' that can be installed. And also it's possible that Tesla has a retrofit kit which improves the current return path for the rotor, maybe end-brushes, but I have no evidence for this.
Bonus -- Sudden Death
For you race car drivers there is another failure mode... The pinion gear will eventually strip its teeth from metal fatigue, trying to withstand the massive forces of moving this 5,000lb car and your fat arse off the start line over, and over again. This IS your fault, although I suspect that most of you are leasing and don't care about the long-term health of your car.