It's likely that your computer needs cleaning. When dust accumulates inside your computer, the CPU will not dissipate heat as efficiently to the heatsink.
This means the CPU and motherboard get hotter, causing the fan to operate. The dust will also accumulate on the blades of the fan, so it doesn't work quite so well and also make the fan unbalanced through having more dust on some blades than on others. Hence it makes more noise because it's vibrating.
So, after all that, your computer probably needs cleaning. To do that, unplug from the mains and take the side/cover off and clean it gently with a new paintbrush, a vacuum cleaner, or an air jet. Pay particular attention to the fans and don't touch the chips with anything conductive such as metal, or your fingers.