between the hd and the reading head of the hd, there is
a layer of ether (1 atom gauge) to protect the contact of the head
with the disk. For some reason (which is very very weird) that
ether layer is damaged. The matter that the head is touching
the hard disk is 100% because of the high rpm of the disks. The
ether layer just makes it more smoothy. So, in your case i guess
that the ether layer is damaged and the head is touching the
disk stronger and makes that noise. I suggest you to get some
backups (as bigbaddog said) and replace the disk. If you don't
want to replace it, make the backups and keep it 'till it die!
good luck