Actually, you've managed to rule out the busboards AND found the culprit. It IS the power supply, and there's something specifically wrong with the second flying bus header!
Next step, therefore: pull the power supply and inspect the "buzzy" bus header. For starters, do you see anything inside the header, such as dust, maybe a tiny bit of metal, bent pin, etc etc? If you can get the supply housing open, do you see anything on the soldered connections to the header that resembles a cold joint, or maybe there's some other soldering fault that's having the same effect as a crossed wire from (best guess here) the Eurorack bus line to a power leg?
Yeah, it's confusing...but this is troubleshooting, that process of elimination of causes of screwy behavior. It's tedious, it's annoying...and it's invaluable, because this is exactly how you learn your rig. Look at it as the 21st century equivalent of being an oboe player and knowing that if you want to play at your best, you need to cut and tie your own reeds. What you're learning as you poke around in this thing WILL translate to being able to deal with other equipment faults in an effective manner, over time.