Dayton Audio 50w T-Amp driven by an iPod.
The tone is 60Hz 21m:30s long on repeat.
Don't ask why that time was chosen, it just feels right to me.

The amp has a volume control. The iPod volume is set to 3/4 and the T-Amp to about 1/2 open. Lowest vibration is just before motor stall.

On the tone tracks, I tried slightly before 90 degress and slightly after, and dead on 90 works best with the least vibtation.