I certainly understand your concern. But actual failures, as in the car won't move, are rare. The milling sound tends to get louder with time. That does not mean that the DU is going to fail in a day, or a week, or in months.The only thing that worries me is that someone on the forum said that the milling noise was a symptom of a drive unit failure later down the road. The milling noise on my car has gotten much worse the last week or so and I'm worried that I'll have a DU failure before my scheduled service visit more than a month from now.
Anyone who states that the milling sound will inevitably result in DU "failure" (car won't move) is speculating.