Apple iMac G5, 20-inch - Manual (Page 271)

iMac G5, 20-inch Troubleshooting - 47 Symptom Charts
No: If the fan activity does not coincide with CPU usage, the sound you're hearing
may not be fan activity. Go to the Next Step.
4. The normal functioning of the hard drive and optical drive will generate additional
whirring and scratching noises that may be audible. We can isolate these noises by
booting the computer to the iMac G5 Install Mac OS X Install Disc 1.
- Place the disc in the drive, and restart your machine, holding down the “C” key as
the machine starts up.
- Once at the Installer window, choose Open Disk Utility from the Installer Menu.
- Once Disk Utility is open, select your Macintosh HD and on the toolbar choose
Unmount. Note: if you have multiple partitions, you should unmount each of these
partitions.
This will spin down the hard drive. The optical drive will also be busy at this time; wait
a moment for the optical drive to spin down also and then listen to the machine.
Is the sound that you've been experiencing still present?
Yes: With the hard drive and optical drive inactive, all you should be hearing are the
fans in the machine. While booted to the CD, these fans should be running at a lower
level since the CPU is under very little task with both drives inactive. Fan sound that
includes objectionable ticking, whistling, or squealing may require further
investigation and/or replacement of the particular fan. Go on to the Next Step.
No: The normal functioning of the hard drive and optical drive will generate additional
whirring and scratching sounds that may be audible. All of these sounds are normal
and do not indicate a failure with your machine. If you wish to check the health of
your hard drive, see article 152349 - Replacing a disk before it fails.