The idea is to have a pendulum generating random sounds while it bounces off the edges of the plate.
The plate would have some sound plates on the edge with different tones so when the pendulum hits them a nice soft sound would come out.
The way to make the pendulum move all the time would be by placing a magnet under the plate that repels the pendulum, this way the pendulum can keep moving around for a very long period.
A much sofisticated version includes an array of electromagnets that would force the pendulum to move in that direction. This could be used to play predefined tunes and reproduce music.