I reckon you'll have to go through the experimentation of trying different things to try to pinpoint what the cause is, and how then to eliminate it.
I'm hoping other Rhea owners will chime in, and give you better advice than I can, but probably the basic things like where the unit is placed in regards to your other equipment (EMI), or RFI interference may be a cause?
Is this noise a hum? A hiss?
The only other things that come to mind is ground loop issues, or that the Rhea you have is somehow sensitive to "dirty" AC Mains power?
Sorry that I can't offer much more help than these suggestions, and I do hope you get this problem resolved. Mark