All you need is something out of balance. In the gemstone polishing industry we use an old motor mounted under a tray with a collar on the motor shaft with a bolt sticking out one side of the collar to provide the out of balance. The tray is vibration mounted using rubbers such as shock absorber rubbers and they run like that for years and years with very little in the way of noise, especially using things like old washing machine motors that are synchronous rather than commutator style motors such as you would find in sanders or drills.
