Cant believe the guys who say their iMacs are quiet and can't hear them on a silent room...
iMacs are very noisy due to their HDDs. The fans are extraordinary silent, thats true. You won't hear them unless watching lots of youtube videos and flash and stuff. The 3.5 7.200rpm drives inside the iMacs ARE VERY NOISY. I have a 2009 27 inch which I had to return 3 times because I though the HD was wrong or something for being SO NOISY.
After those 3 I understood it was because they are 3.5 7.200rpm drives. I famed from laptops, which are 2.5 5.400rpm, which is a lot let noisy. I even returned my iMac more times due to the yellow tint issue and dead pixels. Guess what? Those others were still very noisy, and I am in a very quiet room. The noise was so loud you cannot even work. Every time you open an app or every couple seconds you could hear the drive spinning. Horrible.
So no, iMacs are not silent. Their HDDs are the main problem. Of course, as of 2010, iMacs can be bought with internal SSD, so that way you can avoid the noise and then yes, your iMac will be very quiet, as is mine thanks to a SSD I bought. It was the only solution.
iMacs are not silent people! Stop misleading.