It's not clear from your description if the rumble is coming from acoustic feedback (through the air or turntable plinth) or from the mechanical parts that make up and run the platter. I think you're leaning toward the latter as the source of the noise but I'm not sure.
Besides the motor & its mounting, have you changed the idler wheel? Rubber parts can become hard with age and this will transmit a lot more noise. The spindle for the idler wheel could also be a source.
The platter bearing, spindle and spindle cylinder can also wear with use. If these parts are no longer to spec, the loose or irregular fit can cause low frequency noise.
The rubber wheel shouldn't cost a lot, but if the platter assembly itself is worn, you may be moving into "not worth it to fix it" territory. There are some pretty good performing current generation turntables that don't cost an arm to buy new.
Good luck with your search for a fix.
Besides the motor & its mounting, have you changed the idler wheel? Rubber parts can become hard with age and this will transmit a lot more noise. The spindle for the idler wheel could also be a source.
The platter bearing, spindle and spindle cylinder can also wear with use. If these parts are no longer to spec, the loose or irregular fit can cause low frequency noise.
The rubber wheel shouldn't cost a lot, but if the platter assembly itself is worn, you may be moving into "not worth it to fix it" territory. There are some pretty good performing current generation turntables that don't cost an arm to buy new.
Good luck with your search for a fix.