Electrical buzz AFTER shutdown Mac

Hi, everyone
I have a really weird and disturbing problem with my 24" aluminium 2008 iMac.
I recently discovered (before updating to SL) that I after shutdown I can hear a faint electrical (kind of high pitched) buzz coming from the back of the iMac, from inside it, basically. More specific, from the left side of slot that handles the ventilation. I suspected that happened because the plug I used was not grounded so I plugged the iMac into a grounded one. Still hearing the same worrying noise. When you're in front of it, you don't hear it at all, you have to have your ear close to the ventilation shaft in the back to actually hear it. The electrical buzz stops (obviously) when the iMac is unplugged and returns the moment it's plugged again. Am I the only one having this problem?
Any suggestions, please? What could it be?
Thanks in advance, I look forward to your feedback.

Interesting. Never heard of that before. The only thing I would do is to repair permissions using Disc Utility/run Techtool and run the AHT (Apple Hardware Test).
http://support.apple.com/kb/HT1883

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    Message was edited by:
    pmcs

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