After upgrading to Lion the fan of my MacBook Air (2.13 GHz Intel Core 2 Duo) went crazy most of the times: It kicked in almost immediately after start up. The wierd thing was that none of the processes were consuming high CPU power.
After trying out a couple of things (disabled flash, deactivated all apps in the start up folder, deactivated spotlight etc.) I thought about the kernel: Snow Leopard booted the 32bit kernel by default but Lion now boots the 64bit one. Could this be the reason which made the CPU so hot and kicked in the fan so fast?
I reverted it back to the 32bit mode and …voila! Quite life is back! Now let’s see if the 32bit mode has any disadvantages over the 64bit one. Time will show…
Here is how to change the mode:
To select the 64-bit kernel for the current startup disk, use the following command in Terminal:
sudo systemsetup -setkernelbootarchitecture x86_64
To select the 32-bit kernel for the current startup disk, use the following command in Terminal:
sudo systemsetup -setkernelbootarchitecture i386
More in Apple’s Knowledgebase: http://support.apple.com/kb/ht3773
UPDATE: I highly recommend using SMCFanControl in addition. This little widget controls the minimum speed of the fan. I have set it to 3000rpm which is higher than Apple’s default. Consequence is that the MBA stays longer cool and the fan noise is still reduced to a minimum.