See official Apple technote here Note that this article is limited specifically to the late 2009 21 and 27 inch iMacs - not earlier models. And it doesn't explain the discrepancy of user reports that claim their hard drives are quiet, particularly the allegedly silent Western Digital 2tb Drives versus the 1tb Seagate digestive noise machines. I'm not buying a blanket statement that hard drive noise is "normal," particularly when having previously owned 2006, 2007, and 2008 iMacs, the hard drive noise was virtually undetectable - unless you put a stethoscope somewhere on the machine. Obviously I have the 1tb Seagate grumbler, but in every other respect, my [replacement] 27 is perfect - no yellow tint issues, no flicker, etc., so I'm not inclined to pursue the issue. It's not like it's that loud, where anytime I operate the machine, the neighbors knock on my door and ask me to keep the noise down. You kind of get used to it after awhile. Still, from a machine whose latest design represents, according to Jonathan Ives, Apple's best thinking and innovation, it's a curious step backwards for Apple to officially recognize that these machines are nosier than previous generations. Let's see if Seagate is invited back to the party for the next refresh.