About 3 or 4 days ago my hard drive (I think) has started to make a whirring/revving noise at startup. It starts slow, then gets faster and faster until it is like a motorbike maxed out, then the noise just stops and all is silent. Sometimes it takes longer to stop, and sometimes it stops right at the login screen. What could be causing this? Maybe it is something else, but the noise seems to be a hard drive sort of noise and is coming from the general area of the hard drive... so what else could it be?