the last time this happened to me was in the yr 2000 , the hdd would make noise like 2 metal plates are rotating in opposite directions and grinding each other , its somehow the same case here , Now - its just about time the noise will increase AND the drive might start to get 'physical' scratches on the platter.....back up everything ASAP....if in warranty RMA , if not then wait for it to go kaput and u get a bahana to get a new one hyeah:
that scratching noise is commonly reffered to as the "buzz of death"...hyeah:
what i feel is that a head crah is occuring,,ie the read write head of ur hard disk drive is touching its rotating platter
get it replaced soon as others have suggested .....eventually it will die..very soon......Following a head crash, particles of material scraped free of the drive surface greatly increase the chances of further head crashes or damage to the platters...