OK, that's very clear. And very weird.

The part I really don't get is that it hums both with and without the power cord. It's easy to come up with explanations for one or the other but not both.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8