Vintage DD turntables. Are we living dangerously?

 try this Lew: flip the table upside down and hold it. You'll feel the motor drop down; flip it right side up. Let it settle and then run it to see if the noise persists. That has worked for me before.

That screw we are referring to is actually a hollow cup into which they put some kind of plastic material.  A bearing ball rests on top of the plastic. You can see a pic of the ball on my system page. I conjecture, assuming there is not something terribly wrong with your motor, that the bearing ball is not properly seated on the plastic--that's why flipping it can help. How it became that way is anyone's guess. It may have something to do with the condition of the oil supporting that bearing ball. If the well is bone dry then then your finding that greater weight on the spindle leads to more noise makes sense. For all we know, the noise might be the ball grinding away dry against its plastic seat.

But get in there and see what's going on. In addition to relubing the ball (even though my platter spun smoothly, the well was more or less dry, it spins much more smoothly after a relube), you'll want to check on the condition of that plastic seat and of the bearing ball. I don't know whether anything can be done to replace the plastic (maybe Peter, with his fabrication skills, can make new ones for all of us :) ), but that bearing ball can be replaced with a SiNi one. I've done it and so has Aigenga iirc.

Lewm
  I second the recommendation to remove the bearing nut and inspect
the oil condition along with the  [plastic]  wear plate.

In other tables the wear plate is often changed due to dimpling
and the resultant change in tolerance. 
 

Totem et al, I would not be surprised if wear on the thrust plate has everything to do with my problem.  Tonight I removed the motor entirely from the chassis.  Then with the motor on my workbench, I was indeed able to confirm my hypothesis that pressing on the spindle while spinning it could reproduce the rubbing noise.  With my stethoscope I can pretty much say for sure that the rubbing is NOT coming from the bearing per se, however.  It's coming from frictional contact between a circular gray metal disc at the top of the motor structure (the first thing you see if you remove the black motor cover) and the green circular PCB that lies directly underneath the gray disc.  The green PCB is fastened to a brass platform by 6 screws. I perceive that pulling up on the spindle also pulls up on the gray disc; they're attached as one piece.  This creates enough space between the gray disc and the green PCB such that the noise goes away entirely.  Thus, I think the "cure" for this is to gently turn the screw at the bottom of the bearing well in a clockwise direction so as to push up on the spindle, because of its contact with the thrust plate, and create enough space to cancel the rubbing.  Any comments on this idea would be appreciated; I am not sure of the function of the gray disc/green PCB arrangement.  However it seems they must together form some sort of sensor for speed, because the pieces are not nearly hefty enough to be the stator and rotor, which I can see reside on levels below this top tier.  If they are a sensor element, then I fear the possible problem that might arise from upsetting their spatial relationship.  I sent a photo to JP and Peter; I haven't the time right now to post the photo on the internet so I can cite it here.

At this point, I see no absolute need to mess with the lubricant. Unless someone has an opinion to the contrary and a reason to back it up.

rw and totem,
Last night I removed the screw cap on the bearing housing and inspected the bearing.  There is a very tiny dimple, probably about 1-2 millimeter in diameter, in the center of the teflon thrust plate, representing minimal wear after 30+ years, in my opinion.  The bearing itself looks OK without removing it, which I am loathe to do.  I will take a look at it with a magnifier to be sure it's ok.  The bearing oil looks only a little bit dirty. I wiped it out and soaked it up as much as I could with lens tissue, chosen for its lack of shredding, so it won't leave little shards of paper in the well. I will replace the original lube with either Technics oil which I ordered from KAB or Red Line 0W20 synthetic motor oil, the recommended lube for my L07D bearing.  I don't see any reason to change the thrust plate or the bearing itself, pending my close-up inspection of the latter. Then I will just screw in the cap a little farther than it was, so as to raise the spindle and the gray metal disc along with it, creating separation between it and the green PCB.  That's the plan.

Last night I changed the lube to Red Line 0W20, and everything seems good, once I fine tuned the adjustment of the screw cap. In my unit, there is a range of adjustment that is less than a quarter-turn (<90 degrees) between the unacceptable conditions of the gray disc rubbing against the top cover of the motor (spindle too high) and the gray disc rubbing against the PCB (spindle too low). Since the pitch of the thread in the cap is not steep, this seems too narrow for long term comfort, but we will see what transpires with continued use. I would rather not have to mess with the screw adjustment on a frequent basis.  I let everything settle overnight with platter, platter mat, and record weight in place over the spindle, before I will re-install the motor.  Just in case.

For what it’s worth, the platter definitely takes longer to coast to a stop with the Red Line oil in there than it did with the old factory oil. I’m not claiming that this matters at all.