|Could be a faulty fan (or even something touching the blades). Unless it is coming out of the speakers it is unlikely to be software. You need to try to determine exactly where it is coming from - which is not always so easy. Don't forget that there is a fan inside the PSU. Fans sometimes respond to a drop of thin oil on the bearings.|
If it is the HD I'm surprised that you have run into it again. Have you tried disconnecting the HD? It all depends on the type of sound it makes - not very easy to describe on here.
We all live on the same ball.