A previous hint discussed how to make hard drives quieter, or to stop them 'clunking.' Unfortunately, the solutions provided were not easily automated, and wouldn't last across reboots.
This AppleScript allows you to automate the solution, change the settings, and launch on reboot. First, you need APM Tuner X 1.0a2 saved in your Utilities folder. Cut and paste the linked source code into Script Editor. Use File -> Save As, and set the File Type to Application Bundle. Uncheck "Startup Screen." Put the file somewhere useful, e.g. ~Applications.
Run the new application. Voila -- new power settings, quieter drive! You can change the power management setting. The default of 220 is reported to silence the continuous 'clunk' of some drives. The application can also be added to your Login Items / Startup Items, via the Accounts Pane in System Preferences. Successful completion and some errors are logged to the Console. It is 10.3, 10.4 and Universal safe. Check the code for more comments. Sorry for the sucky formatting.
[robg adds: APM Tuner X has been abandoned by its developer -- it was freeware, and supposedly released to the community, so I think it's OK to tell you that I found at least one working download link in this thread. Note that I haven't tested either APM Tuner X or this script.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061004170017725