Last night my fan kicked into high gear all of a sudden. I shut it down overnight and when booting up this AM I find it still running on high...quire noisy, of course. Is there something overheating? How to make it run at normal speed? Ambient temps have not changed...80 deg. 20% CPU usage. 20 in. iMac 7,1, 2.4 ghz, 4 GB memory, etc.