Hi Rene
You will notice that an SMC reset can help with fan issues, see http://support.apple.com/kb/HT3964
Fans
The computer's fans run at high speed although the computer is not experiencing heavy usage and is properly ventilated.
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Have you ever installed software, if you remember that might affect fans or increase their speed etc.?
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Remove the following file, com.apple.PowerManagement.plist from /Library/Preferences/SystemConfiguration
-you will need to enter your admin password to remove it and you will need to restart the mac.
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I would investigate your computer's performance with Activity Monitor. It is possible that you have an application that is consuming a lot of CPU and therefore increasing fan speed.
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You could even try the following test(s)
1) If you are on Lion, start up the mac with command-R to enter the recovery partition. The nice thing with this is that it's just a stock basic OS that starts up the mac, if there is no fan noise, well we could argue that your mac is probably ok from a hardware perspective because the software running is very basic, just the basic OS.
2) You could go farther and install a clean OS on an external drive. Granted, this would not be your software setup but it would help rule things out further in terms of a hardware or software problem.
3) Step 2 above is probably the most effective but none of your software would be there but it's nice test bed. Same Apple hardware and software that shipped out of the box.
4) Listen for any noise, might it be your hard drive that is dying and starting to cause heat, thereby increasing fan speed, Disk Utility can show you the SMART status of your internal drive – open Disk Utilty, select Macintosh HD and you will find SMART status, if it reads anything other than passed, the HD is reporting some issue to Disk Utility. If this is the case, backup (which you should always be doing anyway) and replace the drive.
Are there other symptoms along with fan noise? Is there a noticeable heat increase somewhere specific on the machine?
When troubelshooting, reduce complexity. Connect as few devices as possible, load as little software as possible.
Maybe your software is not uptodate?
Isolate the issue, install OSX on an extrenal drive and update it. Does the issue persist?
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If you do decide to get it looked at, consider contacting AppleCare instead, they will diagnose over the phone, your call to them is not very expensive, they will charge for the phone call but it's for one issue for 30 days. It might be cheaper than taking it in for service.
The nice thing about AppleCare is you can ask related questions along the way, they can be very helpful. Let them know if you don't understand something, they are very patient.
I can help you further if you wish, just let me know.