Why Is My Computer Fan So Loud?
Usually dust buildup restricting airflow, which forces the fan to spin faster to keep temperatures down. It can also mean the computer is under more load than usual, or that a fan bearing is wearing out and making genuine mechanical noise rather than just airflow noise.
The common, easy explanation
Dust inside the case or laptop restricts airflow, so the fan has to work harder — and get louder — to maintain the same cooling. This tends to develop gradually over months or years.
When it's a specific fault
If the noise is a grinding, rattling or clicking sound rather than just louder airflow, that points to a fan bearing or blade issue rather than simple dust, and is worth having looked at before it leads to overheating.