VPI Aries 1 Motor Hum?

I just inherited a beautiful Aries 1 from my uncle and I noticed that the motor puts out a low frequency hum. It does't come out of the speakers or anything audible like that and you can't hear it 4 ft away but I'm just curious if anyone else has had this experience?
I only had to reposition the motor on the shelf and it went away. Weird how just a position on a shelf can make it stop oscillating. 
It probably did not stop vibrating; instead the vibrations became inaudible when you re-positioned it on the shelf, perhaps because the shelf does not produce sympathetic vibrations when the motor is in its new position.  Just as good.