August 4th, 2007 by 
Christopher Thompson While on the phone with AppleCare on Friday I attempted to record, via GarageBand through the internal microphone, the sound of the fans chirping. I believe I was successful. Keep in mind, this recording was accomplished by using the internal microphone, not an external mic. It stands in stark contrast to the beauty and utility of the Intel based iMac and has exhibited itself on every single one I’ve owned over the past 13-14 months. Be aware that this sound is not related to the hard drive or power supply as might be suggested based on past experience with the G5 tower.
recorded on Friday, August 3, 2007 - Fans Chirping
February 17th, 2008 at 9:19 am
Hello, I have an iMac C2D too (bought in november 2006) and it’s been exhibiting the same problem on and off. It will do it for a few weeks, then stop for a few months. When it does, it’s unbearable.
Do you have any newer information about the problem? I think it’s the hard drive fan in my case, because using smcFanControl to speed that one up to the maximum seems to curb it for a few minutes, but then it begins again. Thank you very much.