Thanks for creating this thread - I was going to create the same one tonight..
I received my second i5 iMac on Monday, and it is going back due to a lot of yellow on the screen.. a large yellow band going down the middle, and am starting to notice yellow creeping up from the bottom. But.. I am also having the same problem you mention..
This is actually my fourth i5; my first two had the 1tb Seagate drive and I had terrible rumbling from the Seagate drives, but it was not the steady constant noise that you mention.. So I replaced the second 1tb iMac with a 2tb iMac, and my first replacement (sent back for yellow) and now my second have had the issue you describe. Here is how it goes:
I walk up to my quiet sleeping iMac and wake it up
It is quiet, the screen lights up, and then I hear something start in the machine.. I know it sounds funny, but sounds like an airplane engine warming up
Then I hear the constant noise.. Kind of a hum, but not so much a hum when putting it on a mouse pad (as mentioned in this thread) but a steady noise..
When I feel the chin of the machine it has a steady minor vibration the whole time, and it almost sounds like a quiet version of the superdrive spinning.. It definitely sounds like something is spinning up and spinning the entire time I am using the computer...
It is kind of like white noise, and I do get used to it as it is not that loud, but it is noticeable..
Here is the worst part.. when I wake up my machine, nothing I select (Safari, Itunes, etc.) responds until the noise warms up and runs constantly.. Usually takes 20-25 seconds for Safari to respond (and the noise to be running) from the point of the screen appearing from sleep and I select Safari at that point.. I literally need to wait for the noise to start before anything I have selected launches..
I downloaded iStat Pro and the fans seem to be running at normal speed.. Honestly can't tell if it is a fan or something else spinning...
I have a note into the Senior Apple Advisor I was working with on my last iMac to report the yellow screen, he seemed genuinely surprised that I received a yellow tinge iMac (-:, however I have not heard back from him since Tuesday and I am going to contact him tomorrow for a full refund. This is it. Can't keep getting these things and running into more problems and getting disappointed..
In the meantime will continue to use my properly working 6 year old G4 Powerbook, wait for the next revision of the iMac and see how it operates..
One thing also to mention, I did not notice this noise at all with my 2 1tb Seagate drive iMacs, only on my 2 2tb Hitatchi drive macs..