Probably the sound of your carbon brushes arcing and burning up! -
But, all kidding aside, the Vboxes and Gboxes of the world probably put out 50 volts unloaded.
When the relays click in to drive the motor, there's no telling how much the voltage drags down (or doesn't)
And it's not well filtered, either (if at all).
So, if you have a 4Dtv receiver, try that for grins...
It puts out 24 volts, and I'll bet my long green/blue tail, it's both well regulated, and well filtered!