Many ways to make a fan more quiet. First, it is necessary to know why it became loud. Many questions, beginning with:
- what kind of PC?
- which fan? power supply, cpu, gpu, case?
- what does it sound like now and how does that differ from before?
- did something else happen at the time the fan sound changed?
If it is a particularly loud sound - or change of sound to buzzing or screeching or beeping or clicking - then it may be covered by warranty.
Recording the sound may be helpful - I use the microphone part of my webcam for that. Audacity is the name of a good piece of recording software if you need a program with which to record.
https://www.techspot.com/downloads/3194-audacity.html
A new fan is inexpensive, but figuring which, and why, and then replacing it can be costly if you take it to a shop for repair outside of warranty.