So if I understood right, sound quality is not affected, just that a power frequency related (e.g.120 Hz in the US) noise is somehow superimposed to the audio signal. In that case, it is most likely a connection (usually connections to "ground" related issue. If there are nothing visibly loose or opened, such as wires coming off, then it could be some bad solder joints that have gone from weak to poor, and could be hard to find by the naked eyes.\n\nYes, it certainly does sound as if there could be a break in the internal ground plane. However failing power supply caps can cause this issue also, so there is excess ripple from the power supply. A scope on the power supply rails would show this problem fast.