There is nothing really to wear out on the motors themselves APART from the bearings.
Even good quality bearings need replacing - in fact T-Motor themselves suggest replacing bearings after approx 60 hours flight time !
What you are hearing is no doubt a bearing that has or is running dry (although I would expect more heat build up). Be warned that this bearing WILL fail it is just a matter of when. If it seizes your Phantom will drop from the sky so I would suggest replacing the entire set of 8 (2 per motor). It's not a difficult job as long as you take your time.
With regards to periodic lubrication, I would say the answer is YES. I have been flying collective pitch heli's for years and years and I have NEVER lost a bearing to failure. I put this down to the fact I regularly lube all my bearings about every 20 flights with a drop of oil (Labelle 108 or similar).
There will be people that appear and say 'Never lube - the bearings are sealed' There will be people that say 'If you lube you will attract dust and that will make your bearing fail'. there will be people that say 'If you lube your quadcopter will spontaneously erupt into a fireball and fall from the sky like a meteor'
However.... I have spoken personally to one of the major European importers of T-Motors over in Poland and he has had these conversations directly with T-Motor the manufacturer. They directly advocate and advise to GENTLY lubricate bearings every so often to prolong the life and keep things running as they should. They also recommend complete replacement every 60 hours of flight time.
I hope that helps.