Hello all,
While I've been reading all of these suggestions as they came through, it's not until now that I'm replying. Sorry. I am dumbfounded by all the the great ideas and thought people have put into helping me with my problem -- THANK YOU! So... just yesterday afternoon I removed and reinstalled the cassette. I also added a small touch of grease to the splines, as suggested. I wasn't able to take it out do to a day full of rain, but I hope to tomorrow. I'll report back on my findings.
I ended up just making the one "adjustment" so I would be able to determine if it, and not any other adjustments, was the real solution. I fit doesn't work, I'll move onto the BB.
To answer a few of the questions here..... all components including chain are in great, lightly used, condition. This is the bike that sees the least use, even more so after a year of the pandemic, and I would imagine that everything has less than 1,000 miles on it. I don't keep track of my mileage, and am a really bad estimator of it, so it could be even quite a bit less than this. Definitely not more.
I did remove and reinsert the seat post, and loosen and re-tighten the saddle bolt. This has traditionally been the number one source of mysterious noises for me, but my fiddling had no effect this time. Which I kind of suspected, given that the noises also occur when I'm out of the saddle.
Regarding the construction of the cassette... the 3 largest cogs are indeed on a "sub cluster assembly," which I found to be quite interesting given that it is only these three that make this noise. Seems like the rivets or just the construction of this part as a whole, could be the culprit, as suggested. Maybe my locking was a little on the loose side, and maybe there was some play between this cluster and the rest of the cogs? I sure hope so!
Anyways, that you all for your time and the thoughtful replies -- they mean a lot!
I have another, unrelated question that I'll ask in a separate post. See you there, I hope!