Insidious cycle? WTH are you talking about? I don't exactly enjoy lugging a 27" computer back to the store to deal with not-so-Geniuses.
The previous two units were returned for light bleed which, being a build quality issue outside of my control, trumps the buzzing issue entirely and thus my clean power "doubt" didn't need to be challenged with respect to those units.
This unit is nearly pristine and only has the buzzing, which all three 2012 iMacs. You're right, I acknowledge the power could very well be dirty. It's suspect #1 but I have not had the opportunity to verify it yet other than ruling out my UPS as being culprit. I'll likely pick up some model of line-conditioning UPS.
If you re read your own posts your focus remains on the buzzing issue in all of them. Returning a computer 3 times for a buzzing issue without verifying an acknowledged power compromise is.....insidious.