I have the same problem on an older lab supply.
As I understand it, the varnish (between the transformer plates) wears out or becomes brittle and then they start to hum.
But it is fixable, if you're willing to set up your own jig to impregnate them (it doesn't sound super hard, but it requires having a vacuum pump and bell jar- and its potentially smelly if you use an oven to bake dry it afterward.) or pay a bit for it (sounds preferable if its available).
I have not inquired as to how much this costs but I plan to, if the isolation approach doesn't quiet it enough.
Some people also seem to have had an experience where they performed some intervention (re-varnishing) to fix it, which seemed unsuccessful at first in curing it, but over time, afterwards, with use, the transformer became much less noisy.