OK, if it is the HDD that is causing the sound, why does it seem like it is randomly happening? I really can't see a specific reason which would trigger these clicks and sounds.Sometimes I wonder if the piezoelectric disk beeper makes such noise or if there's a mechanical relay I never noticed for power. If not, than the HDDs make the sound. I'm quiet sure the noise is caused by the HDDs only and not by anything else. If you hear the click sometimes, than it's ok, this happens to my machine since years, when the drives were new and still today. If you get the click noise very often, especially when the drives spin up, than the drives, resp. one drive will die soon.
I don't have a choice but to neglect these random clicks I guess if I want to go with Debian...