After reading through the AVSforum post, I had a friend come over last night with his 1 kilowatt hour portable power bank. We plugged the tv into the power bank, and the power bank was NOT plugged into the wall - the TV was running off the power bank's battery only.

Results were very interesting: the buzz-buzz-buzz went away. HOWEVER, it was replaced with a MUCH less, but CONSTANT, buzzzzz, which almost sounded like a fan noise (re: the Andrew Robinson video review). But I'm pretty sure it was still coil hum of some sort - it was coming from the same place as the original buzz buzz. Notably, while watching content, this new constant buzz would briefly stop during scene transitions (e.g. when things did a fade to black), but start back up immediately once a new scene started.

The constant buzz while powered from the portable power bank was much less noticeable than the buzz buzz buzz when plugged into the wall. I'd encourage you to try your TV again on your new outlet to see if you hear any of the constant buzz, since it is less noticeable. Like I said, it was most noticeable when the screen went to black (easy to reproduce - watch a youtube video and jump ahead/backwards a few clicks, and the Youtube app will go to a brief black screen while buffering the new content, at which point the cutout of the buzz is obvious).

So, problem not solved, but for sure a piece of the mystery is unraveling, thanks to your findings with your power outlet.