A colleague mailed me the following how-to. I thought that it was too good to keep this for myself. This how-to is for the people that have iMac G5s. This problem is very common and can appear “out of the blue.” The fans will start producing a big noise and it only stops once you reboot your system. As you can imagine, this is very annoying! This how-to could help to solve this problem.

The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any power issue, resetting the SMU may resolve it. The SMU controls several functions, including:

• Telling the computer when to turn on, turn off, sleep, wake, idle, and so on.

• Handling system resets from various commands.
• Controlling the fans.

Note that resetting the SMU does not reset the PRAM. Resetting the SMU will not resolve issues in which your computer is unresponsive - in these situations, restarting your computer will generally suffice.

Resetting the SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your computer still exhibits these types of issues even after you’ve restarted the computer, try resetting the SMU. To reset the SMU on one of these iMacs:

1. Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.

2. Unplug all cables from the computer, including the power cord.
3. Wait 10 seconds.
4. Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
5. Let go of the power button.
6. Press the power button once more to start up your iMac.

Unlike earlier iMac G5 computer models, the iMac G5 (Ambient Light Sensor) and iMac G5 (iSight) computers do not have a button on the main logic board for resetting the SMU. This eliminates the need to remove the computer’s back cover to reset the SMU.