Some people have reported that dust and dirt get stuck on the fan blades causing it to "rumble" as it turns. You might try cleaning the existing fan to see if it helps improve the noise level. Others have just replaced the existing fan or added another to supplement airflow. Using a "better quality fan probably won't improve airflow, but will often reduce the noise level. I believe the size is 120mm, and be sure to order the 12 volt version. Fans are available from a number of sources such as EBay, Amazon, etc.
Here's one example:
https://www.amazon.com/Cooler-Master...s=computer+fan