Well, here's where I am. I have verified that none of the external portions of any cable is causing this sound. All the front spokes/nipples appear fine. There's nothing bouncing around within the wheel. QR seems normal. It's not not the vale nut and I don't think it's the vale either, as it is snugly pushed against the aperture all the way around. I'll still go to the hardware store tomorrow and get some electrical tape and try that trick.
I've carefully tapped the entire frame. It all sounds the same. Took a 1000-lumen light and studied every millimeter of it. Nada. The crankshaft bolts are solid as a rock. Both derailleurs and the hangar are fine. I cannot reproduce the sound by playing with the saddle or seatpost or rear light setup. Went over the headset again. It seems solid.
What I seem to be left with is the good idea one of you had about an internal cable rattling. And the hubs. So, that's my next project.
I do still find it interesting that it seems to only be really prominent when in the higher (harder) gears. I'm not smart enough to know what that means. Is there something wrong with my chain? This def does not sound like chain rub/FD stuff to me. As I say, it actually sounds like a giant Jew's Harp, something between rattling and buzzing.
Will post more after the next set of tests. Thanks to all very much for your time and feedback. Really appreciate it.