Well, there are two flying busboards attached to the Meanwell. But thank you for directing my attention thataway, because this morning I finally made some progress towards isolating the problem and solving it.
I attached a row of relatively power-hungry modules to just the top busboard and tested it out. Def a bit more ground noise than I'd like, leading me to think a PSU and overall busboard upgrade/reconfiguration may be a priority regardless, but -- no buzzing/interference sound, and generally work-with-able.
When I attached the same set of modules to just the bottom busboard, I did get the interference sound. Strangely it didn't sound quite the same as what I recorded, but was still hugely noticeable in contrast to the first busboard.
So, my thought is to find I fresh flying bus (I think I have one somewhere) and replace that flying bus. Hopefully that will work and if so this setup will be usable enough until I can afford a more complete overhaul.