I had a Seagate HD do the click of death a few times on me a few weeks ago. I couldn't get it to fail any of SeaTool's tests except the long generic once... So I contacted Seagate and told them all the troubleshooting I had done (external enclosure, external dock, internal) and that it clicked at one time or another in all 3 scenarios but I couldn't get it to fail a diagnostic test. They agreed to RMA it based on it clicking.
So, if your drive is under warranty, do a live chat or email them (depending on the manufacturer) and see if they'll RMA it for you. In any case, I'd be backing up the really important data on it now if you've got any place to store it.