The fan on your computer is getting unusually loud for a multitude of reasons. Generally, fans become louder when they are running faster to help cool down the system. This typically happens when the computer is hotter than normal due to a lack of ventilation, excessive dust or dirt build up, too much strain on the hardware, an overheating component, or a failing cooling system.
A lack of proper ventilation is generally the most common cause for a fan becoming unusually loud. Fans need adequate airflow to work properly, so if your computer is not in an adequately ventilated area (e.g. near an open window, or on a desk with plenty of open space), the fan will have to work harder to pull in cooler air from outside the case. This will then cause it to run faster and become louder.
The second most frequent cause for a loud fan is dust and dirt buildup. Dust, dirt, and pet hair can easily get sucked into the fans and clog up their blades, reducing air flow and causing the fans to run faster and become louder. To reduce the buildup of dust, you should regularly open up the computer case and use compressed air to blow out any dust or dirt that has accumulated on the blades.
Another potential cause of loud fans is the strain on the hardware. If you are playing demanding games or performing intensive tasks, then this can push the components in your system to their limits. This can cause them to heat up and run at higher temperatures which can force the fan to speed up in order to cool them down.
An overheating component can also lead to a loud fan. Some components such as the CPU, GPU, and hard drive can overheat when they are being used heavily or there is inadequate cooling. When this occurs, the fan will have to run faster to try to cool them down, causing it to become louder.
Finally, a failing cooling system can also be the cause of a loud fan. Cooling systems can fail over time due to general wear and tear or due to a manufacturing defect. If your cooling system is failing, then it won’t be able to adequately cool your components, forcing the fan to run faster in order to try and compensate. This will make the fan louder than normal.
If your fan is getting unusually loud, then you should check to see if any of the above issues are present and take the appropriate steps to correct them. If the noise persists, then you may need to replace the fan as it may be defective.