Repair diagnosis for cdp with humming tranny

Transformers hum because they DO. Loose windings, poorly made, nothing really can be done electrically. but you might try tightening up the frame of the transformer. even adding clamps to the frame (if you can) or isolate it. Rubber under it, around it. Seal just the transformer in a canister somehow. or just dampen it with something pressing physically on it.             
How hot does it get is the only reason to back off on dampening material.Even Blu Tack type putty added around it. Polyurethane for bathroom cracks outdoor cracks.. But that stuff is permanent. So be sure it ain't gonna over heat before you glop it up with polyurethane.  
I could see making a card box to fit over it, fill it upside down with transformer in the polyurethane, let it st a week. peel off the card.

Just teach him the lyrics.

Try it in another home. 

If it goes away, you have DC on your line. 

The cure for this is to eliminate the source (often digital power supplies, and dimmers) or to get a balanced power supply. 

Best,

E

Over time, the transformer core adhesive starts to break apart, maybe some large heavy duty hose clamps could help.
https://www.qsl.net/kf8od/ccore11.jpg
https://www.qsl.net/kf8od/ccore7.jpg

Thanks for the advice. Unfortunately nothing works, really. 

Elizabeth, FYI as you like using spinners and DAC's. I tried using the digital output of the Raysonic into the Marantz (SA11s3) input - didn't care for the sound at all. But, I acquired two other cheap DACs, the Cambridge and the Peachtree. The Cambridge was OK but it really worked better with some cheap spinners I have. The Peachtree was an improvement over the built in DAC in the Raysonic. Clear, clean, linear, and tonally spot on. Great synergy with my system. FWIW.