I came across a hard drive yesterday (an old maxtor 250gb) that was refusing to mount and making some very, very strange noises. Since it's all backed up, I had no qualms about opening it to see what was going on.
Having it spin up while open answered some of my questions:
If only that sound was a little more rhythmic, you could do great things with it.
But the platters: the platters were crazy. magnetic material had been spread wildly over the whole drive, and had interesting sandy patterns on the disks themselves. The inner section of the platter is deeply marked from the read head scraping it.
And best of all, it happened to ALL THREE platters. The exact same markings, on both sides of the platters. I've dismantled quite a few damaged drives, but none presenting like this.
And then we get to the read heads in the drive itself - And suddenly it becomes more clear. Look at the bottom head, there - that's incredibly badly out of place.
I'm hesitant to say whether the bottom read head could have caused all that by being obviously out of place (and rubbing against the motor, causing some of that noise). I'm guessing that a serious physical jolt knocked the whole armature slightly out of alignment and gradually led to this.