Thanks very much, that is encouraging. I tightened the plastic nut as much as I dared, so I will try Bikerduud's suggestion. Is there any trick to taking off the platter? Does it unscrew or just slide off?
The only thing that I've ever had go afoul in nearly 270,000 cleaning cycles with the HW-16.5 is that the screws that hold the platter shaft to the motor arbor worked loose. If the other suggestions made here for things to check to solve the problem don't work, try checking this.
You would need to tip the machine on its side and remove the bottom panel to do so. If they are loose, be sure to orient the shaft straight and vertical before you tighten them. You may also want to put a drop of Loctite on each screw before you tighten them.
Osage Audio Products, LLC
One other tip, while you're at it. The vacuum tube can be adjusted and the slot should not point straight down (180 degree position). Per instructions I received from VPI, it should be at the 185 degree position when viewed from the spindle. Thus it will be slightly "open" to the film of liquid as the platter rotates. Also the tube should not extend more than 1/16" beyond the edge of the record.
I bought my 16.5 new and it was out of adjustment from the factory. A friend had even more problems with his new unit until it was adjusted per these instructions.