Ok I might have done something a tad wrong. I recently replaced a failed hdd in my late 2009 aluminum 20 inch. I watched countless teardowns and hdd replacement videos and picture guides before I did this at it was my first mac machine. Not every time, but occasionally when I restart the machine the temp will be some where in between 79- 86 degrees f and the fan will be going at 1500rpm randomly for about 30 seconds and then stops. Other than that everything has been flawless since the replacement. Did I do something wrong? I put the thermister correctly on the hdd when I reassembled. What could be causing this as before I replaced the hdd I never heard the fan at all. Mac was bought on august 2, 2009 when hdd died I tried to have apple exchange it, but the wanted to give me a refurbished imac. Since I have had terrible luck with any refurb in the past I just replaced the hdd in it myself.