Thanks for your feedback! Yep, desk surface is clean, and it started right out of the box, so it was not a scenario where it was working properly and then began to collect dust and dirt..
What is interesting is this noise was consistent on BOTH of my 2tb iMacs, but did not occur on either of my 1tb iMacs.. When I spoke with the Senior Apple Technical Advisor regarding my first 2tb iMac, he thought it was a bad sensor, but to happen on two separate iMacs? Hmm...
I own an iMac 27", Core i7, upgraded to 8GB of memory. Mine is dead silent when working on it, but I hear the airflow very silently when I play games on it (on Win7 bootcamp).
It was more noisy at the beginning as I heard the hum of the hardrive. Remember those are 7200rpm drives, so be careful how you put your iMac on your desk: if you put them on hard desks the Hd "hum" will be emphasized by the desk as its being transported right through the iMac desk stand. My solution: I put a very flat mousepad underneath and bingo, it was gone.
Another tip: when your imac is standing close to the wall put a post-it note on the wall about 10 inches above the heat slit (where the air is fanned out). When the fans ramp up due to heat you will see the post-it note flapping in the "air", so you know they are spinning on full (mine do when I play heavy duty 3d games).
Otherwise I never notice the machine is turned on. I love this one, best investment so far as I can run both worlds on a fast machine.