Sometimes after running a system update, be it a security patch, an incremental OS update, or a full upgrade to a new OS version, you may experience problems with the system's fans running loudly. This may happen immediately and continuously after updating the system, or it may be an intermittent but frequent occurrence. Sometimes this can be coupled with system slowdowns, but at other times it may seem to happen with no other apparent effects.
There are several reasons why this can happen, including corrupt hardware settings, corruption in software configurations and temporary files, and a fault in the OS upgrade or update itself.
The first thing to do if the fans on your system are running loudly is to check the system for high temperature. Feel around the chassis, and on top of the keyboard for laptops, and if the system is abnormally hot in one area then those components are being taxed for some reason and are generating a lot of heat. If this is the case then it is likely the system's hardware is running fine and you have a problem with how the new software is being run. Therefore, go to the "Activity Monitor" application, ensure you are viewing "All Processes," and sort them by "%CPU" to see if any are using a lot of processing time. If so then you know the issue is with your software configuration and you can likely avoid having to reset hardware settings as a troubleshooting step.
If any programs are using a high percentage of CPU and also have an icon next to their name, then go to that program and quit it to see if the issue goes away (if the program's name is colored red then force-quit it using the "Quit Process" button at the top of the window). Sometimes user applications can have incompatibilities with new OS versions that can cause these problems, and may benefit from running a general maintenance routine on the system (focusing on removing user caches), reinstalling the application, or locating and removing the application's preference file.
Various system components such as the CPU, graphics card, logic board, and hard drive have temperature sensors on them. The temperatures are constantly read by the system management controller (SMC), which tries to keep them cool by increasing fan speed for these respective components as they get hot. Settings for this feedback system are stored in hardware, and are updated when the system undergoes low-level changes such as adding a new hardware component or updating core system software.
If the SMC settings are at all corrupted then the SMC will not properly respond to temperature changes, and may crank up the fans when they are not needed. To correct this, the easiest solution is to reset the system's hardware settings, starting with the SMC itself and then going to the PRAM if needed.
Resetting the SMC on Mac systems will differ for each model. In most cases it involves unplugging the system and pressing the power button for a few seconds, but in other cases you might need to hold other key combinations. Apple has most of these steps outlined in a knowledgebase article on managing and resetting the System Management Controller.
To reset the PRAM on your system, reboot the computer and hold down the Option-Command-P-R keys at the same time immediately after hearing the boot chimes. Allow the system to cycle through a couple of resets with these keys held, and then release the keys and allow the system to boot normally (it may take a few seconds longer to boot after this reset).
Temporary file corruption
Many times odd system behavior after an OS update can be attributed to cache and temporary file corruption. When the system is running, the OS will store commonly used extensions and other code in caches, which are accessed at boot and when the user opens applications. There are numerous caches on the system, including boot and caches, caches for system files, fonts, user applications, and browsers.
If you are experiencing a hardware problem like fans ramping up, then one possibility may be that a corruption in a system or boot cache might be to blame. In these cases, first try rebooting in Safe Mode (hold the Shift key immediately after the boot chimes during start-up) and then use a maintenance tool to clear the caches. Optionally you can run a full general maintenance routine on your system; however, be sure the maintenance tool you are using is compatible with your system. Many times when Apple has released a full OS upgrade (such as the upcoming OS X 10.7 Lion release), support for the new OS in various maintenance programs may lag, and there is potential you might do more harm than good by using them on the new system software.
As one quick step beside using a maintenance tool, you can force the system to rebuild the boot cache by booting into Safe Mode and then opening the terminal and running the following command. This will tell the system that the extensions folder has been updated, which will cause the current boot caches to be refreshed when the system is next restarted.
sudo touch /System/Library/Extensions
Faulty OS upgrade
The last possibility for fans running loudly could be if your system software became corrupted when updating or upgrading the system. Though this is rare, sometimes if people have reset their computers, experienced a power outage, or have had existing system software corruption on their system when updating then the new software can end up being corrupted. This can also happen if there are existing directory structure or partition issues.
In these instances, first boot to the OS X installation DVD and use Disk Utility to check and repair the boot drive. If you have a third-party disk management tool then you might also use it to check and verify the boot drive. If there are problems, consider performing a full repartition and format of the boot drive (including writing all data to zeros to help manage bad blocks), but be sure you have a full backup of your system and data before you do this (a file-level clone or Time Machine backup should suffice).
With the drive checked out, try reinstalling the OS or if you did not format the drive then try reapplying the update for your system. Apple should have all updates available for download at its support site. If you are reapplying an incremental system update, be sure to use the Combo version of the update, which will include all changed files since the initial OS release and therefore has a better chance of clearing the corrupt files off your system. At the very worst you can perform a full reinstallation of the OS, which should replace all system files fairly seamlessly, and then update it. A full reinstallation will ensure all system files are free of corruption.