Repair diagnosis for cdp with humming tranny

I have an older Raysonic CDP which has a humming transformer once the unit has heated up for an hour or so. I pulled the tubes, hooked it up to a a DAC, and it works perfectly (never hums) if you like the sound, :-(

I've had it into a local repair shop twice - they tried but didn't fix. Any techies out there who might have a thought?