I have a 250GB Seagate Barracuda 7200.10 for over a year and a half now, and it made that dreaded clicking noice once every now and then. It started a few weekes after I got it, but all the diagnostic tools showed the drive ok.
The Last few days it started doing the same noise but about 4 or five times in a row 3 or four times a day.
again SMART is OK and Seatools' short and long tests both pass. and there were no lockups that could be attributed to the drive failing. drive runs at 42C idle.
The only recent change is a complete re-org of data on my disk using Disktrix defrag. I put all the archived files in the center of the drive and the program files, games and windows on the outside.
How worried should I be that a failure is imminent? Should I trust the diagnostic tools and ignore the clicking sounds?
Fill out the web form for warranty replacement on Seagate's site. It will issue you an RMA number and will give you instructions for sending the old drive back. Then they will send you a new drive (be aware, it's not actually new, it's factory refurbished, but it will be fully tested and come with a warranty, usually the balance of the warranty remaining on your old drive).
Optionally, you can pay $19.99 to have advanced replacement, where they send you the replacement drive first and then you send them the old drive back.
The web site will ask you what's wrong with the drive, just put that it's making clicking noises. The web site will still issue you an RMA no matter what you put.
A a Hard Disk warms-up the platters expand (slightly), this causes tracking errors, so most (if not all) drives will re-calibrate the heads when tracking errors ocour, some times this is heard as a "chirp" some times as a "click"; if the drive is over a year old and isn't reporting errors I'd say keep using it, but make sure you have all your data backed-up (just in case).