Hurst motor rumble?

I have an HW19-IV with the stand alone motor.
When I drop the stylus onto a stationary object next to the platter, there is rumble as soon as the motor is turned on. Turn it off, the rumble goes away.

I removed the drive belt and tried again. No rumble. This means the noise is not coming through the platform on which everything sits.

Next, drive belt still disconnected,I dropped the stylus and spun the platter by hand. No rumble. (The bearing appears to me to be tight and well lubed so I don't think the noise can stem from disaligning of the platter bearing by the pressure of the belt.)

It seems obvious that the noise is coming from the motor through the drive belt to the platter. I am using a standard Hurst 600 rpm motor.

I always thought, apparently erroneously, that the drive belt itself damped motor noise. If any of you has a belt drive turntable, I would like to know what happens when you perform the aforementioned test. You need deep bass and may have to turn up the gain.
Can anyone verify that a "quieter" motor might reduce or even eliminate this rumble?
Has anyone done actual measurements of this effect and what are they?