My iMac has suddenly started to run the built in cooling fan on almost a constant basis. I vacuumed the tiny vents along the bottom thoroughly. Also vacuumed the exhaust vents and there is no heat in the exhaust - just nice cool air. Sometimes it will quit running after a while and other times it puffs off and on. The other night I turned it off completely so I know it was cold by morning but as soon as I brought it up the fan started running. Other than the VERY annoying sound is there anything to worry about? Or is there some fix I can look into getting? Thanks!!!