Jack at the Guardian explains why you PC may be whirring and what to do about it:
Unexplained whirring is usually due to excessive use of the central processing unit (CPU), which creates heat and noise, and slows down or even stops any programs that you actually want to run. It’s therefore a good idea to find the cause and eliminate it. To do this yourself means delving into Windows’ internal processes, but it’s useful to know how.
this is easily the most profound post ever.