@stratamania @Rick Found this. Had never heard of this phenomenon before, but now everything makes sense.
Basically, looks like I cut the nut slot a bit too deep. I stuck a piece of paper in the nut slot as described in the video, and voila buzz is gone. Adding relief also eliminates the buzz as I'd discovered.
In efforts to fix the buzzy low F note, I had lowered the nut slot a bit, which then introduced this back buzz. The open low E string is not especially buzzy, at least not enough to make me suspect the nut slot. The back buzz is super metallic and either sounds like hardware rattling or actually causes the hardware to rattle a bit, which led me on the wild goose chase.
I can maybe fill the nut slot for now, but I'll want to get a new nut long term. I'll bring to a pro.