You've likely already done so, but if not, back up whatever's on that HD immediately!!! Healthy HDs generally don't make unexpected noises. (Can you check that HD's S.M.A.R.T. status in Disk Utility, or, even better, with a robust 3rd party app, like TechTool Pro?)
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
