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Over the past couple of weeks, I've noticed that my macbook's fan ALWAYS kicks on after a few minutes of inactivity. It doesn't seem to matter how many programs I have open or what I was doing previously. If I let my comp site idle for a few minutes, I will hear the fan spin up. I can confirm that something was going on by looking at the CPU usage graph from activity monitor and the CPU graphs from iStat menus. Once I start using my mac again the CPU usage drops and the fans return to their usual 2000rpm. Is this just the OS doing stuff once it detects that I'm not doing anything? Has anyone else experienced this? Can anyone offer an actual explanation of what's going on?

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You don't have some 3D screensaver set up to start after a few minutes? :) – Jonik Sep 28 '09 at 6:42
I was using the "Arabesque" screensaver. After I switched to a less flashy screen saver (switched to "Computer Name") and the problem seems to have gone away! :) I guess that was it...if you want to post that as an actual answer I'll upvote and accept it. Thanks for the suggestion! – Derek Swingley Sep 28 '09 at 7:13
Done. :) Heh, that was an easy one then - glad that it helped. – Jonik Sep 28 '09 at 8:19
up vote 3 down vote accepted

Kind of obvious, but... You don't happen to have some 3D screensaver set up to start after a few minutes? :)

I used to have that issue with my MacBook so I ended up switching to a more lightweight "screensaver", i.e. blank screen.

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