Best guess is that it's the CPU fan or a case fan. If it was the hard drive i'd expect that you'd hear it from time to time when the PC is running.
If you, open up the PC case and switch it on (be careful!). You should be able to feel a vibration relating to the noise if it's the CPU fan.
Changing the fan is a DIY job if you are careful. If unsure then it shouldn't cost much to sort out at your local independant service center (iro £20).