From your description, and your comment to @Randolf's answer, it sounds to me like your drive is suffering from a heat-related issue, and is definitely in need of replacement.
In addition to echoing the advice you've already received - backup, now - I'll add that it is probably worth doing everything practical to keep the drive as cool as possible.
- Turn off your computer and allow it to cool for at least an hour.
- Find some way to significantly increase the airflow over your disk:
- Sometimes just removing a panel from your case and directing a large desk-fan at the drive enclosure is enough.
- Depending on the enclosure, you may need to temporarily remove the disk - if you do this, take appropriate precautions, ie:
- Protect your system from static discharge
- Ensure your disk is kept level at all times while it is running
- Do not touch the disk while it is running
- While ensuring the disk is receiving very good airflow, power on and take your backup.
- Once the backup is complete, power down and store your suspect disk safely, now you should work only from your backup data.
- Once you are certain you have all of your data safely recovered, if you have any personal information (or other sensitive data), destroy the suspect disk. In any event, dispose if the suspect disk responsibly.
My reasoning for this suggestion is largely based on the symptoms you reported, the 'clack' sounds may be related to a form of 'thermal recalibration' (which only some drives report correctly through SMART), the screeching sound may indicate that the platter speed dropped too far to sustain head-flight correctly and the drive did not have time to properly unload the heads, which would indicate a motor / bearing failure.
In any case, a good, complete, backup has to be your top priority.
+1 for this answer. Particularly interesting to me was the point about the thermal recalibration feature being a possible source of the "clacking" noise. – Randolf Richardson – 2011-07-24T00:23:56.657