Grinding noise coming from one motor

The students are running the drone motors, and one of them is making a grinding noise. We swapped the motor out with one from last year, and it is doing the same thing.

What could be the issue?

Hey emoons,

I’m really sorry to hear that you’re going through this. Could you do us a favor and send a short video that captures the sound? This would help us get a clear idea of what’s going wrong and find the best way to help you out.