I had 3 iMac's with dog whistle and buzzing noises, got my forth now, and there's no buzzing, and almost no dog whistle, just slightly. But I got this constant humming noise, even when not making the HDD work. When I put the iMac to sleep, it goes away, but starts after a while. Don't think it's the HDD and not the fans either, cause they make another type of noise. I don't recall my other 3 iMac's I sent back had this humming noise. What is this humming noise? Is it a hardware or software problem? Is it common?