the fan is running really loud on my MacBook Pro

It is either overheating or it thinks it is overheating. Have you been doing CPU intensive activity such as gaming?

You can try:

Reset the System Management Controller (SMC) on your Mac - https://support.apple.com/HT201295 - Learn when and how to reset the System Management Controller (SMC) on an Intel-based Mac.

Try running the Apple Hardware Test or Apple Diagnostics depending on the age of the computer. If a sensor isn't responding correctly, then the fans will run at high speed to protect the system.

You can also use Activity Monitor located in the /Applications/Utilities/ folder to see if you have excessive CPU utilization.

If the laptop is still supported, then you can also take it to an Apple Store or Apple Authorized Service Provider to be checked out.