The Tomboy Gnome applet for writing small notes is responsible for writing every 20 seconds or so to the hard disk. This periodic input is responsible for premature wear and tear of some hard disks in newer laptops. I have identified this bug and notified the Fedora development team. Fedora 9 places Tomboy by default in the Gnome panel. If you use a newer laptop with an up-to-date Linux distribution and hear clicking more than twice per minute when the machine is idle, try removing the Tomboy applet from the panel. Hard disk clicking is normal every 1m30s or more. By removing the Tomboy applet I reduced the clicking from three per minute to one every 4 minutes when the machine is completely idle.