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.