Hard to say where the problem is but I would say more likely mechanical as its not constant. Could be bearing related in that it being one direction only thus the screw is moving slightly. Check the coupling and see if you can see and rubbing between the two halves.
Try and locate where the noise is coming from using a mechanics stethoscope if you can get one, if not then a long screwdriver, just make sure you keep any loose clothing away from rotating screws etc when checking..
Hood