That's a bad signal. Back the info up now.
I had a drive start doing that and it completely failed shortly afterwards. Mine was out of warranty.
I found a faq once upon a time..
BASIC HDD FAQ
Rev 1.5, 6 November 2002.
06. CLICK CLICK CLICK or GRIND GRIND GRIND! Whats wrong???
Backup Your data NOW! Noises such as these means that your drive has failed, or is about to.