Eh ... I spoke too soon.
I'm really stumped now. So after that last post, I replaced the 2nd flying bus with a fresh spare one I had, thinking I had it in the bag now -- but when I added my 2nd row of modules back in ... once again, I got the buzzing.
This leads me back to wondering if it's the PSU somehow, and related to a certain amount of power draw. But if so, why on earth does it have no issue running the same exactly modules on one flying busboard, but not another? Unless I have the bad luck of the spare one also being bad somehow? I might have one other, maybe I'll have to try that. It just makes no sense to me.