Earlier I noticed a faint noise coming from the top left of the iMac, a little 'hiss' like, but otherwise hard to describe. I presume it's the HD, but I'm not quite sure as diagrams show the HD being more-a-less in the middle. Perhaps the room has got quieter, but then again the concern is that the iMac has got louder. Any possible explanations or experiences? Any help would be greatly appreciate, thanks.