10db buzzing sound MC12000 after it warms up

Maybe a MC12000 question, maybe not… I’ve developed a 10db buzz only on the channels I have connected to the MC12000.  Interestingly, makes zero difference if the component is powered or not.  Also makes no difference turning up the volume or placing the unit on Mute… its a static 10db buzz. Even more interesting, two of of the three components are connected to the DA2 via optical, the third is my MT5 and yes, I did check the grounding wire but again, the buzz is only present on these three selections…Amp is completely dead quite otherwise.  Any thoughts, considerations, simular experience???  Thanks for sharing, greatly appreciated.