Is it MC or is there some correlation between the number of people in the house and what you're hearing. More people - more background noise.
What are the specs on the PSU? If it pushes out low ripple voltage, then it would not feature as a likely culprit.
What other stray current sources could be an issue?
At home, my main unit gets its power via a UPS, then a power conditioner with AVR to deliver A.C. power @ +/-1% and then an HD PLEX 100W fanless PSU (I think the spec is +/-1mV or something cool sounding). But this is really for reliability and to deal with the effects of solar PV on the grid boosting voltage beyond the nominal spec - from what I know, this gives electronics an easier life and is better for their longevity. I went for the HiFi spec PSU because I just wanted to when the previous unit died. Personally I can't hear a difference between that unit and the Id I've got in my man cave where it gets UPS power only - so no AVR and power via the OEM wall wart.