Isn't rumble referring to the bearing effects (the quality of the bearing design and manufacture)? Wow and flutter are the primary parameters affected by belt vs DD.
Direct Drive Rumble
I remember back in the 70s when direct drive was an upgrade from belt drive in every manufacturers lineup. This was before Linn, SOTA, etc. took over the world of turntables. Conventional wisdom has it that direct drives couldn't compete because they were noisier than belts. The theory advanced at the time and still held by many was that the belt mitigated the influence of motor vibration on the whole system. What I recall, however, is that there were consistently lower rumble figures for the DD tables over the numbers posted by the belt drive units. Is there another measurement besides rumble to indicate motor vibration and its deleterious effects?