Over the past 15 years I've handled a huge amount of bad hard drives and many times the most obvious indicator is the sound that it makes. One data recovery company has put together a list of sounds from failing hard drives from different vendors. This list also indicates what the actual cause of failure that is producing that sound. Those of us in the 2K Techroom have heard many of these sounds directly in the course of our workday. Now you can hear these sounds as well by going to the website linked below.
Click here for more details.
Oh - and those with iPhones/iPads/iPods Touch can also download an app from iTunes that visually demonstrates (and audibly as well) some of these hard drive failures.