If it used to be quiet and is now noisy, there are several things to check.
Firstly,
clean it out. Dust makes fans louder, also dust raises temperatures which can cause fans to spin faster.
If that doesn't help, it's likely a fan is failing (a failing fan usually makes a grinding noise).
The best way to see which fan it is would be to open up the machine and start it up whilst listening to the fans. If you can't tell, use a cotton bud/q-tip or whatever you want to call it to
briefly stop each fan in turn. Obviously when the noise stops, you know which fan is noisy and needs replacing.
Fortunately fans are easy and cheap to replace - the worst case scenario is that it's the power supply fan which is more involved, but cross that bridge if you have to.
Hope this helps.
