Good to know?
Odds are you are still downloading the block chain and once that is done it won't access your hard drive much more.
Regardless, a clicking hard drive is a bad sign. You might want to invest in a new one.
When I download something in firefox it doesn't click, so it's not a function of how much data is being written.
It could be that you have bad sectors that were allocated to Bitcoin-related files. Bad sectors often cause clicking, and they could be assigned to any file, not just Bitcoin, just as a matter of probability.
I'd have to hear the clicking to know for sure. But Bitcoin is not a hard-drive-intensive application. I suggest backing up important files and at least running a diagnostic to make sure the hard drive isn't on its last leg. (hard drives use forward error correction to silently recover from the beginnings of failure, then they offer a diagnostic feature called S.M.A.R.T. so you can catch the failure before the whole drive is ruined)