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!