As of April, 2015, what are the current official drive unit failure symptoms that one would experience prior to a drive unit failure? And what are the detection methods Tesla currently uses to define drive unit failure or predict drive unit failure? I'm asking for updated clarification since in the past we've heard that possible symptoms are "clunks", " whines ", and "humming/buzzing". I would think that Tesla now has certain metrics to measure against rather than relying on owner feedback that may be falsy induced or inaccurately observed. Thanks ahead of time.