Horrendous clunking noises from a hard drive are always a very bad sign. Back up your important data immediately. It would probably be worth downloading and running the hard drive manufacturer's diagnostic utility (they run from floppy, there is no need to boot into Windows). Here are the most common ones:

Seagate Seatools

IBM / Hitachi Drive Fitness Test

Maxtor Maxblast 3

Maxtor PowerMax

Western Digital Software & Drivers

Fujitsu Utilities