2012 iMac still on, even when shut down?

"noise came back on. i am taking it to genius bar as planned."

Dr. John's "Surefire Fix" to solve your problems…

Just buy a nice power strip or surge suppressor and plug the Mac and related peripherals into it (as I do).

When you shut down, just reach down and turn the power strip OFF.

When you want to reboot, reach down and turn the power strip ON.

Do this, and I _guarantee_ that you will NEVER be bothered by such problems again!

It could be "coil whine", magnetic vibration of a loose or poorly potted coil, transformer, etc. in the power supply which may be temperature sensitive (cooler in evening?). I'm sure that would pass their diagnostics as long as the voltages were within tolerance.

Or perhaps your hearing is better in the evening?

Did the Apple techs actually hear what you are hearing? I don't hear anything from mine, but my high frequency hearing isn't all that good anymore.

If you are still within the return period, that might be an option. Then just buy another one, although they may be hard to get right now.

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I would imagine the power supply is still "on" even when off as it has to power the "wake on LAN", "wake on Bluetooth", monitor the power switch, etc.

PavelT said:

so i bought a APC voltage regulator, and plugged in my imac. and...the buzzing continues...

a recording of the electrical noise:
https://soundcloud.com/pavl-1/2012-imac-electrical-noise

Click to expand...

Wow, sounds just like a barking dog. Seriously, it's just the sound of the power inverter for the +5 volt standby power. If it bothers you, switch off the APC power switch. If you bought an APC UPS, I hope it's a full sinewave unit, such as the SmartUPS. The Back-UPS XS and RS units provide a stepped square wave on battery power, which could stress components in your iMac's power supply.