I am using an RPi3b with an IQaudIO DAC+ on board with mpd and upmpdcli as player. Source from lan connection with audio sourced from my NAS served by minimserver. Control point is Android Samsung S6 with BubbleDS.
This has been my standard setup for a while and all is well but in one situation I have an audio hum. I have only noticed this is when I am playing through KEF MUO which is connected by audio plug, (bluetooth not being used,) and with the RPi power plug and the KEF MUO both powered from plug adaptors. The hum is not perceptible when playing but when playing is paused there is a faint hum which increases after playing has been paused for about 10 seconds.
I was about to try unplugging each of the power supplies in turn but this has not been a great success because when I picked up the MUO to hold closer to my ear the playing stopped abruptly. It seems the RPi or the IQaudIO DAC+ is no longer working. I had been thinking earth loops but none of the components are earthed and that includes me! Will do some checking and return but if anybody has similar noise hum issues please share experience of solutions.