OK, a short intermission there due to a bad back.
I removed all the accesories on the front of the engine and ran it for a few minutes. The clicking noise was still presnt, so all front end stuff is excluded.
Next, I took the advice above, removed the oil cap, and stuck my stethoscope down into the cam cover. Sure enough, it's inside there, towards the front, so it's valvetrain noise, I guess either a rocker arm or lifter.
I decided to try one more thing. I changed the oil and filter again. Used synthetic oil and one quart of Marvel Mystery oil. There is little, if any change, so, my final decision is to turn up the radio and drive it 'til it pukes.
. We'll see how long that is.