I had a 2nd hand maxtor that made really loud clunking noises for the 2 years I used it as my main drive - apart from being noisy full stop, I also used to get very loud "thunk" noises, similar to the ones you get when you access a floppy (remember those!)
It never failed (it's sitting on a shelf somewhere) and the SMART stuff never showed up any worrying trends
The really good thing about all this was that it got me thinking about backing up, and I quickly developed a very healthy back up routine
So first - back up you important data, or if you can take an image of the disk, and next install some SMART reading software and monitor the condition. In the meantime keep backing up regularly, which you should be doing anyway