I just got a new K40 laser from Aliexpress and both steppers make a high-pitched whine noise and a grinding sound when operating though it is much worse on the X-axis. I thought the grinding sound was from the gantry moving so I removed the belt from the motor but it still makes the sound. It uses an M3 Nano controller and I’m using MeerK40t.
I did have another K40 but it arrived damaged (hence getting this new one). This first one did not have the high-pitched whine or grinding sound. I tried the M3 Nano board from it with the new machine and had the same issue so ruled out the control board. Don’t know if it’s related but I also noticed that when moving the axes by hand with the machine power down, this new one would jerk a bit and light up the power LED on the main laser power supply but the old one did not. They move smoothly when not plugged into the control board. I used a separate 24 VDC supply to power the steppers but it did not fix the noises so I don’t think it’s an issue with the power supply either. Just turning the steppers by hand slowly I don’t feel any difference at certain steps; they feel smooth through the full rotation.