In particular I'm referencing some of Seagates SATA drives in the ~500GB range. They all seem to make heavy clicking/"ridged" noises (akin to dragging a knife across an old record, only louder?), even when brand new. This doesn't seem to be a sign of wear, but just normal operation. What exactly causes this sound?
Most of the clicking noise is actually from moving the head to an appropriate position or as some sites suggest "hitting an internal stop" which basically mean head reset but moving head rapidly to various positions on the harddrive plate would also cause this noise. Maxtor Drives used to be notoriously noisy in this way.