Most HDs do not have a fan so my guess is either a case fan, the CPU fan or likely the videocard fan. If the inside of your computer generally looks dusty, then you can be pretty sure the fans and their surroundings will have a huge load of caked dust.
You need to establish which fan is the noisy one. Use a soft paintbrush or paper spill to stop the fan momentarily.
Opinions differ about how to clean these fans, my own way is to switch off of course, remove the side of the case and with a vacuum cleaner nozzle close to collect whatever is dislodged as you stroke the blades and every part of the fan and its case.
If you can extract the video card, do so, it will be mush easier to access the naughty bits.
The CPU fan varies enormously from PC to PC, and with all fans and particularly this one, be very careful about what you disturb. Sometimes it cannot be cleaned without removing the CPU itself. That takes you into a much more finicky game which is best avoided if you can.
With luck and an accessible fan, you may be quite startled by the quiet of the computer once more.