Important point from James.
Simplify! Start unplugging interconnect cables and have only one connection to a box/rack that cause a problem. Then unplugging mains cables on the problem box/rack and see what happens.
There are other things that can cause issues. Some devices etc. might do bad things when when the device is not powered on. ... even differential output devices, might not really be, and/or you might not have the common mode rejection you think you do. Narrow it down to actual devices.
BTW time to celebrate again one of the best documents on hum and ground loops:
https://web.mit.edu/jhawk/tmp/p/EST0...ps_handout.pdf