Datacent, a data-recovery house, has a page of recordings of the sound of hard drives failing, segmented by vendor and cause of failure (e.g., "Western Digital 250GB desktop drive with stuck spindle can't spin up, chatters" and "Fujitsu laptop hard drive with bad heads making sweeping sound"). I love the idea of listening to these until you can identify them by rote, like Sherlock Holmes examining a cigar ash or a birder identifying some exotic warbler's song.

More practically, as Datacent notes, "If your hard drive makes noises like these and you are still able to access your files – backup immediately."

Hard drive sounds

(Thanks, Fipi Lele!)

(Image: Dead Laptop HDD, a Creative Commons Attribution Share-Alike (2.0) image from kdga's photostream)