I have virtually no experience with any kind of RC motors, but with electric motors in general, they tend to get noisy as a first sign of failure. So, if any of my phantom motors started to make unusual noises, I'd replace it, if not all four.
Since my last phantom tumbled out of the sky from 200 feet due to a soldering failure, I always start my flights with a stable hover at about 5-10 feet, then hard stick from side to side and fore and aft. Only then do trust the craft to fly with reasonable safety. Still won't fly over people, and only over water rarely.