I have built a cnc with Nema 17 motors. it is controlled by an Openbuild Blackbox. with the external adjustment of volts for the motors. but either the motor stands like it squeaks (easy to hear with a screwdriver on the motor and the ear) and if I turn the volt so it goes away then the motor starts and drives around inside. It does not move on the shaft, but sounds like a jet engine during start-up, again with a screwdriver in the ear Sometimes I can actually hear that whistling sound - howling sound. the motors are at a 48 - 52 degrede when they have been on for some time When I do the same thing on one of my 3d printers, the motors are totally silent. what in the world can it be ??