I wasn't so concerned about this until my wife started complaining about my MacBook being so noisy... The fan didn't used to be always running. I think it was after I upgraded to OS X 10.5 that the fan decided it would just always be running. I have my MacBook on all the time and usually the applications that are open are, Firefox(with 10 active TABs), iTunes, Azureus, VLC and Mail (and Widgets). It used to only come on if Azureus was real busy. Now the fan will start up soon after I boot up and will even keep running when I close all Apps and only have the Finder open. What can I do to regulate my fan? It doesn't seem to want to control itself. Also, my MacBook never feels overheated. I used to have an iBook and that thing got hot. MacBook with constantly running fan fees real cool in comparison so I don't think the fan needs to be constantly running. Any tips or suggestions as to what I can do would be appreciated.