When is hard drive noise bad noise?

I have often heard horror stories of hard drives starting to make noise before they fail. :jason:

I recently had a hard drive die on me, and it had been making a lot of noise before it died. For the past year, it would make this “plop” sound now and again, sounded like the old MAC error sound (remember those days?) but I’m still not sure if that was normal or not.

However, both my laptop’s internal harddrive and a new harddrive I just received yesterday (though, I got it by mail, so it may have been banged around a little, though that is covered by warranty) make a “popping and crackling” noise whenever they are being heavily read or written to.

Is this a warning that the drives are about to fail? Do harddrives usually make a crackling noise during normal function?