Humidity. That's the problem. Not during winter, of course, but it is now hot and humid outdoors and the central air puts out very cold air through the network of ducts that run beneath the floor - which is to say, through the overhead of the basement. "Why is that a problem?" you ask.
Droplets of water on the ductwork, occasionally raining on the basement floor.
I own a de-humidifier, a Midea Model mumble-mumble and yes, I have the owner's manual in hard copy and digital.
So... the manual says to empty the water tank every couple of weeks. Wash it with soapy water, lest it acquire a mold or fungus among us. Guess how often I've emptied and washed the water tank. Did you guess, "never"? If so, you'd win the prize. At least I cleaned the filter. and we could marvel at the dryer lint-like deposits thereon.
Anyway, I have it running again, set to maintain 35% Relative Humidity. like setting the thermostat for 45°F in the summer, the goal is unattainable. I also set it for continuous operation. Now it has achieved 51% (from a start at 72%), and no new beads of "sweat" are forming on the ducts. Problem solved. Maybe.
Saved here for convenience