We are getting to the core of the problem - slowly, but we do.
First, I am fanatic about grounding. I do not tolerate any hum in the system that can be distinctly heard as such and will not cease until I track the source of it and unroot it for good. It is the most detrimental in turntables, as "good enough", not directly audible during record playback, is not nearly enough for best performance. Once one hears it removed, music shines and returning to "good enough" hum performance is painful indeed. Not always, but at times I use batteries to supply the really low noise preamps. And I make sure there are no ground loops possible.
I wrote mechanical noise in operation . That is big difference. Even if you use Thorens Rumpelmesskoppler , which is the best rumble measuring probe known to me, generally producing rumble figure(s) 10 to 20 dB better than even the best of test records, it DOES NOT measure the noise of the turntable/arm/cartridge IN OPERATION. That is to say with recorded signal being played - and bouncing and echoing from the highly resonant stucture called Thorens turntable.
Accelerations of the stylus in the groove can reach 2000 G. That is possible in the treble, roughly 5 to 10 kHz, recording velocities around 100 cm/second. Check any brochure for Shure V15 series, they did provide trackability chart where theorethical and ACTUAL recorded velocities that exceeded theorethical were given. If we take average top quality cartridge available today, with an effective stylus tip mass of 0.25 mg , it is easy to calculate the force acting on the groove walls>record>mat>platter>main bearing>subchassis >etc>etc.
F = m x a, therefore 0.25 mg x 2000 G = roughly 5 or so kilos acting on the groove walls at the high frequencies. This energy has to be dissiapated somewhere, as fast as possible and preferably not be capable of returning to the point of origin, in our case the stylus - or else the stylus will be reading its own echoes for the duration of the entire disc side, masking low level recoded signal. That was meant as mechanical noise in operation. It is here that Thorens in unmodified condition is very poor indeed.
From the point of mere rumble, wow & flutter, it is a very good turntable.