K2CNC Servo Motor twitching when idle

K2 users, I have a KG3925 (parallel interface version) and the Servo Motors seem to twitch when idle. You can hear the audible intermittent noise and if I put my finger on the Z axis motor belt I can feel a little twitch. It is not changing position though. Is this normal? Thanks. (added piece of information, this machine was actually given to me several years ago and the previous owner used it to mill plaster so it was a dusty mess with plaster dust in everything).