Thanks again to the OP for the thread, and to Creativepart for providing the solution. As I mentioned, my Bafang rear hub drive (kit conversion) was doing this. Yesterday, after dousing spoke threads and base of spoke where it protrudes through rim with WD-40, I went over the spokes and snugged all the loose ones. Took it out for a 45 mile ride yesterday, and it was blissfully quiet. With disc brakes, Im not concerned if it's a bit out of true; I didn't notice any issue, and will dismount tire and true it in a few weeks once it's too cold to ride any more.

I'd have figured it out at some point, but you saved me who knows how much time and worry. Cheers!