IBook G4 makes the power-down noise while it's running

Question

I know this is not a good sign and I'm guess that I'm at the early stages of HD failure, but thought it's worth checking. Here's the basic situation:
1. About two or three weeks ago, I noticed that after being idle for a while my computer would make a pinging noise--it's just like the sound you hear when you power down using the power button.
2. The computer didn't freeze or anything so I repaired permissions, verified the disk, had to do some Disk Utility repairs, and went on my merry way.
3. During the past 10 days I've gotten the folder with the flashing question mark and Finder icon a couple of times. The first time I was connected to the network at my office and my iBook booted from there. When I rebooted from the local HD it worked fine. Last night I heard the pinging sound, my computer froze and when I rebooted from my Leopard Disk and went to Disk Utility and the HD didn't appear in the Volumes window. I turned the computer off after this happened a couple more times. This morning I started up fine, was able to SELECT the Leopard DVD from System Preferences and run First Aid on my HD, but n the midst of that, the ping happened and I got the gray "You must restart!" box. Just did that and now my HD doesn't appear in as a disk, volume, or image option.
I'll try Single User Moder, PRam, etc. but I don't think that's the problem I'm having here. Help!

Answer 1

There are a couple of little utilities that can help you keep track of the inner workings of your hard drive and hopefully give you a heads up ahead of a failure:
S.M.A.R.T. Utility
http://www.versiontracker.com/dyn/moreinfo/macosx/32454
S.M.A.R.T. Reporter
http://www.corecode.at/smartreporter/
S.M.A.R.T. Utility can be downloaded and run several times for free. It will give you a more detailed look inside your hard drive than just "verified" or "failing". I think it tends to be a bit more critical than Disk Utility is. If you click on the "Show all" buttons, it will show you any errors on the drive.
S.M.A.R.T. Reporter installs an icon on your menu bar that is green when S.M.A.R.T. status is verified and red when it is not. It's intended to give you a heads up if your drive starts to fail.
Even though all seems well now, I'm not sure your hard drive is really to be trusted, especially if it is getting long in the tooth. It may well work for a while longer, but you might want to monitor how many errors it has and definitely keep everything well backed up.
It could be that your drive is starting to have intermittent mechanical problems, and this is something that Disk Warrior cannot fix. Intermittent problems can come and go and never seem to happen at the shop when you take it in. But if you find anything of concern in S.M.A.R.T. Utility, that would be something more concrete to go on. But as long as you keep everything backed up, there's no reason not to continue using the drive until it goes south for sure.
Good luck!