|All fans start to make a noise after a while|
Its basically dust plus the bearings get worn through age
You need to confirm if the noise is coming from the Power Supply fan (top rear) or an addon fan at the back of the computer
Many times a noisy fan can be helped (quietened) by blowing out all internal dust with a compressed air can bought from a computer/electronics store
You would need to remove all power, and then the side cover, and blow out all the dust (preferably outside at the time)
Every Desktop computer I service I blow out all internal dust
Some have internal dust nearly an inch thick!
If removing all dust does not reduce the noise, then you will need to replace the faulty fan, or Power Supply