I recently helped to produce an on-location livestream concert event that was in a nice home in the SF Bay Area. During sound check, we were getting a lot of hum from one mic and after doing all the stage 1 diags (switch the mic, switch the cable, ground loops, etc.) we gave up for a while and moved on to lighting. It was then that I noticed that there was a dimmer on the ceiling lights. As the owner of the home had used lights with different color temps (don't get me started) I swapped out a few bulbs so that they would match at least. The hum went away. (well, the hum went away as long as the dimmer was at 100%). It was a bulb. They were a mix of CFLs and LEDs. I didn't have time for an experiment and they had a box incandescent bulbs on hand, so we swapped them all out for the duration of the shoot.
So now I've added light bulbs to the list of things that can cause hum.
Hope that helps someone someday.