Well, that’s how I always thought of the noise.
Me: Computer, let’s play CivIII. Here’s the disc.
Computer: Crunch, crunch-- grind, grind. OK, here you go.
Me: Why the hell did you put my settler in the middle of the jungle?
Anyway, what is actually going on inside my hardrive when it makes that noise?
It’s the arm that carries the magnetic pickup moving rapidly across the disk platters to read and / or write data. If it seems to be happening to a excessive extent, you may need to deframent your hard drive. Adding memory would also help as well.
New hard drives have something called “sound management” and can be quite quiet to the point where you may have trouble hearing anything at all.
Unless this is a very old hard drive or you have very sensitive hearing, an audible “grinding” sound does not bode well for the health of most modern hard drives. It might be prudent to back up your critical data.
Is it the movement of the arm, or is the arm grinding the disc that makes the noise? If the arm is grinding into the disc that enthusiastically, why doesn’t the disc wear away with time?
My computer is 4 years old. It always made that sound, as did my 486 before it. My computer at works makes more of a whirring sound, though.
The arm, and its attached read/write head, never makes contact with the disc. If it did, your computer would immediately come to a complete halt, and the hard drive would be toast. The read/write head “floats” just above the surface of the disk, where it can sense, or create, the magnetic information on the disk.
If the head touches the disk, this is called a “head crash.” It rarely happens these days, but was depressingly common in the early days of computing.
It’s kinda hard to see from those pics, but the head does not actually scrape across the disk platter. It actually rides just above the disk platter without actually touching it.
The noise comes from a track to track seek, where the head moves across the disk to find another track to read. When the disk drive reads the data you don’t hear a sound. It’s only when the head changes tracks that you hear the noise. If you listen closely you may hear the noise of the drive motor spinning, but that’s usually drowned out by the sound of the fans in the case.
The noise is made by the mechanism that moves the arm. The head isn’t hitting anything as it moves.
It happened on my iMac last year, destroying every file on my hard drive.
Back up often! (I didn’t.)
Very bad luck, indeed. 
I think that’s the first time I’ve heard about anyone having a head crash in many, many years, and I used to be a sysadmin/LAN manager type person - we didn’t see a head crash on any of several thousand PCs over the course of at least 5 years!
Even though hard drives are very reliable, I’m amazed at how many people don’t routinely make backup copies of their files. I have a little batch file that copies the files that I modify most often (personal financial stuff, my Outlook database, etc.) to my CD-RW, and I just leave a “formatted” CD-RW disk in there all the time. So, whenever I’m leaving the PC, I just hit the icon to run that batch file.
[Dave Barry]
Those sounds are caused by the Whirring-Grinding Unit, or WGU
[/Dave Barry]
From the same site minor provided:
What is the churning sound I hear from my hard drive whenever it is retrieving data?