The Hub

Hello all, got a problem in a telecaster I rewired that has me in a twist.

The basics: friend has a 2001 US Standard telecaster (left handed not that it matters). He bought it used and says it was quiet initially. A few years back he grabbed the pickups out of it and moved them to a new tele body with a torrefied maple neck he preferred. Recently he upgraded the new build with a Lollar CC tele set, and asked me to reinstall the original US pickups into the old US body. At the same time he wanted the cavities shielded and also a 4-way switch added (in the usual configuration). All pretty straight forward.

I did the shielding with copper tape (and checked that the adhesive was conductive). Rewired the pickups with cloth push-back wires (also requested by the friend) and added the second ground from the cover of the neck pickup for the 4-way (removing the jumper). I rewired the harness with the new switch and swapped out the original volume pot which had a mountain of solder on the back, and crackled like crazy even after cleaning it with contact cleaner. Closed it up and discovered the bridge wasn't grounding (no continuity on the multimeter). Added a ground from the back of the bridge to the back of the tone pot. Now when plugged in the guitar has a notable hum. It's quieter than a cable not plugged into the guitar (the classic 60hz hum), and it seems higher pitched. Also, it's present in all four switch positions. Most notably in humbucking, as the friend is pretty sure the set is RWRP. The hum changes slightly in tone in each position, but it's there for them all. And it rolls off with the tone pot (it's high pitched). My suspicion is that I've created a ground loop somehow but I can't figure out where. Is it possible it's the combination of the pickups grounding to both the shielding (via the shielding tape under the pickguard and the bridge, though in the latter case I would have thought that would have prevent needing a dedicated ground wire) as well as directly via their black wires? Or is there another pitfall I'm not seeing? Google has not been helpful, as nearly all the hits are for common ungrounded components. As far as I can tell everything is grounded (continuity from everything metal back to the jack), but the hum persists. I can share pics of anything specific and/or share a recording of the hum in the different positions if that would help.

Thanks in advance!