Sometimes adhesives that bind the stacked plates which are part of the power transformer will weaken and allow vibration. We hear it as a hum. I suspect that is what your friend is hearing. It is a common occasional problem with transformers of any manufacturer.
Knowing that it exists, even if you can't hear it while listening, is annoying. However, as long as the hum is inaudible, it does not degrade the quality of the music you receive from the system. It is not an electrical problem; it is a mechanical one - the unadhered sections of the transformer plates are vibrating.
If adhesive failure is the problem, then when you press down on the top of the power transformer to dampen the vibration,the sound probably will go away. I think the solution is replacement of the power transformer rather than repair of it. The adhesive failure is between the plates and cannot be repaired.
If that is the problem, it requires skill to remove the transformer withour damaging the pads of the printed circuit board (pcb). Don't attempt it unless you are good with a soldering iron.
