I had noticed recently that my Y motor was getting hotter than the rest, and that there was a "singing" noise coming from it when idle. I decided to check out the Vref, and reset it if needed.

As I put my volt meter probe on the adjuster a new sound came from the motor..... perfectly clear reception of a local AM radio station! Now to be fair this station is pumping out 50,000 watts less than a kilometre away, even so, I wasn't really expecting to hear it. I continued to adjust Vref between the recommended values of .39 - .5 v, but everywhere in that range the motor still made a sound when idle, the sound would change in tone depending on Vref. I had to go as low as .31v to silence the motor, at this value the bed seems to move OK with no skipping, so I'll probably leave it there and see what happens. As a matter of interest I tried the probe on the other 4 adjustment points and there was no radio reception audible, and all Vref values were in range. I am wondering whether I might have a suspect motot or driver board.