Explain the sound you heard....?
Any failure of any kind will vary with the operators flight habits. I can use any of 100's of examples, but just take one where the pilot lives in very humid climate. Like the kind where you see fog from it be so humid. I would have to think that alone would reduce the lifespan of the motors. Or take the guy the was out to break their distance record, knowing he'll get great reception with low cloud cover from a possible rain storm. Then is two miles from home point and has no idea it has started to rain in the location his quad is currently at. And last one,.....And then you have the pilot that lives near the ocean and goes the the beach from time to time to meet up with his bro's to fly along the beach front where occasional saltwater mist can come into play.
My feeling is the motors are going to be what gets the most wear over anything else. If DJI would allow us to see our max RPMso that we could rate the performance drop off it would beneficial to this question.