It sounds like a bird tweeting, but its a very regular sound, although it does stop sometimes. I think it did it ages ago, and then disappeared, but its come back now. I sent the iMac to sleep and it stopped. Woke it up, and it started again. I guess its a moving part, just not sure if its likely to be a fan, or maybe the hard drive? I cant really pinpoint where it is coming from, but it might be the right side as you face it? Anyone able to help?