Aragon 24k humming on left channel

My system is based on this Aragon 24k feeding into an Aragon 4004 Power. Lately is has developed a ground noise or hum on the left channel. Very noticeable with the phono input, much less with all the line ones but still there. It also increases when you put it in the “mute” position. I have heard that it could be caused by a capacitor or capacitors gone bad at the pre side. I also heard that it could come from the external power supply. Music quality is pristine as it has always been for the last 20 years that I had it, but I would like to take care of this if possible. Any help is highly welcome.