http://www.tacktech.com/display.cfm?ttid=287
Use the above link for HDD diagnostic tools.
Your drive is on the way out. Back up any critical data ASAP. If the clicking is too persistant to work with let the computer cool down then try again. If it still doesn't give you enough time. Place the drive in a zip lock bag and put it in the freezer for about an hour. Then place it immediately in the computer. This may give you enough time.