iMac has a buzzing noise on the top back left corner

"This seems to be a ""known issue"" as they say in the Mac World. Could be the LED Backlight Board or the PSU. Since there's been quite a bit of chatter about this I would go to Apple Support and see if the machine in questoin is still under warranty, if not, there may have been a silent recall, extended warranty or coverage still in effect on your machine. If there doesn't seem to be, I would contact Apple directly and rais a bit of fuss- as there's quit a bit of this on the Discussion Board and other Mac Forums.

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You must have had a repair receipt with the description of the problem. I'd go back and up the chain to the manager, or contact apple support directly. They took the machine in, if there was nothing wrong why would they do that, and keep it 9 days? Something smells... don't let it go out of warranty like this.

"I had the same problem and after 1 to 2 months of buzzing I turned it off and it would not start again! I am fairly certain that this is related to the PSU and I have already ordered one (I am based in Greece and I do all my work on my iMac so I can't afford to waste time with the fairly low quality Mac service available here).

This may or may not be relevant: I have been using the iMac with a 2nd monitor attached. The GPU has been getting pretty hot and I assume that this has placed a strain on the PSU.

Update: I decided to power the iMac with a UPS. For some bizarre reason the humming stopped immediately. It has been a month now with the UPS and the buzzing only happened once. I have ordered (and received) a new PSU from ifixit, which is sitting in my drawer for now."

I had the same buzzing noise issue with my mid-2011 27" iMac. Then one day I shut it down to move it and it was totally dead when I hit the power button. I replaced the PSU with a new one and the buzzing stopped – for about two weeks and then it started up again (with the new PSU unit in). It does the increase/decrease volume with brightness adjustment. I even tried lifting it off the desk to see if it was a vibration as some people had stated with no success. Seriously, this issue is all over the internet and there is no definitive answer/fix from Apple!

After that I replaced power supply unit at Apple service center the noise totally gone. By the way, old SATA HD's makes some noise too.

"My 27 inch 2011 iMac started to buzz like this after 5 years. It was silent for the first 5 years.

Then the buzz started. I changed the PSU. The buzz came back after 2 days. Then, I changed the LED backlight board and the buzz came back after an hour...

There is nothing left to change! WTF. If someone finds a fix, please let us know."