technology, sound, and design
On a system running Debian 8, it was observed that HGST TravelStar 7K1000 hard drives would continuously make a faint clicking sound while idle -- roughly one click per second. Not quite sure why they do it, but the solution to eliminating the clicks involves changing the Advanced Power Management setting for each drive as such, where 'x' in 'sdx' corresponds to the appropriate drive:
# hdparm -B128 /dev/sdx