!!!iMac 24" HDD strange chilling sound

I searched the forum for some kind of answer but I havent found anyone with THIS particular sound coming from iMac.
I have brand new iMac 24" and a few days ago I noticed really chilling sound coming from it. It sounds like high pitched, high frequency sound kinda like that piiiiiiiiiip in your ears comming from nowhere but your head or when you hear car-breaks disks squealing while breaking. Sometimes its just that, not very often, but sometimes its louder and more pronounced just as a thin metal plates spinning and touching would make (or at least thats what I imagine they would sound). It comes suddenly, something like ziiiiiiiiiiiiiiiiiiiiiip! And than nothing, until it comes again. I checked the temperature its about 55-58 deg. celsius. Other than that, the Mac is working great but it really scares the crap out of me when I hear it because it feels like the whole HDD will just explode or just die with a sudden screech. Any experinces, ideas?

I tested fans, theyre fine. The sound is definitly coming from hdd but everything is still working fine. Even when i hear the sound there is no change in performance (at least not that i can notice). i think i´m taking it in tomorrow. I´ll post if I have some answers.

Thanks for the support. Weird thing, though, this iMac i have: I´ve been paying extra attention to that sound whole weekend while working. I tried making hdd work as mush as possible, but the sound came maybe 2-3 times. Unbelievable. And days before weekend i heard it almost 2-3 times/hour. It got scared Does anybody know some app/something that would put extra pressure on hdd, you know, writing/reading while sending heads from edge to the center etc.?

Ok, just got back from the store. Guy said it shouldn´t sound like that of course, just as you guys said. He mentioned taking the mac in, dispatching it to tech guys, changing hdd etc... so it got me thinking: I´ve had this mac for a week now and it came this way, I mean, the hdd noises didn´t start after some work, they came few hours after it was un-boxed. I don´t want a brand new-used iMac! I don´t want some weird guy who never saw this kind of machine (btw, I live in Croatia, so people don´t get to repair iMacs! in best case they repair some trash can PCs) meddling with it and returning it to me as a not-new hardware. Maybe I´m being nuts, but although they have Apple service here i just don´t believe that they know how to do it, simply because there arent enough Alu iMacs sold here. Let alone the ones in need of a repair! So, do you think I can demand a new one, because they gave me a faulty one or am I stuck?

Just to finish the thread... got brand new iMac from the dealer, everything is great now, silent as it should be