Computer clicking/ticking noises?

I know that the clicking/ticking noises I hear in some computers must be the hard drive. (some computers are way louder than others especially older ones) However I'm wondering why or what causes the hard drive to do this? The noise I can best describe is the sound you would get from crackling something or frying eggs at a very high temperature meaning there are like dozens to hundreds of clicks a second. I ume it's the mechanical arm of the hard drive going crazy but that can't be a good thing if done continuously? Despite defraging and cleaing up junk files, some hard drives keep doing that. Why? Does it mean the hard drive is getting read to die?