Constant audible disk usage?

[cacoe]

I can hear my hard drives ticking away somewhat crazy constantly in Windows 8, I can't figure out what's causing it though! It's not the corrupt dying hard drive sound that you might least want to hear, it's a genuine "something heavy is at work" sound but task manager only reports the "system" service as using any disk when idle and it's using a max of 1MB (not sure if it's read or write) but this surely shouldn't cause such apparently usage?!