I got my new iMac last Thursday and all day Thursday I didn't hear any noise coming from the iMac when it was just idling... during the night (I leave it on) it was almost entirely silent. But since Friday night it's had the fan whirling away when idle and it literally is doing nothing. It doesn't seem to want to slow down. I can't imagine why a quiet almost silent fan on the first day is now noisy... I did add an extra 16GB RAM in on the second day but that's all so why is it noisy mow?