NEVER EVER turn your computer's fans OFF !
Your computer is probably overheating, causing the fan do go into high speed to compensate.
Either the air flow is obstructed, the heat sink loaded with dust, or the fan need to be replaced.
Turn the computer OFF and UNPLUG it.
Open the case.
Take a look at the main CPU fan and the heat sink under it.
The heat sink is probably filled with dust.
Use a can of compressed air to blow that dust off.
It's adviseable to perform that operation outside or with a working vacuum cleaner nearby BUT NOT reatching into the computer's case.
Take a good look at the fan itself. It may be covered with fine dust. Using a #8 to 10 artist brush, gently brush the fan to remove that dust. It may unbalance the fan, causing it to vibrate excessively.
Reconnect the computer, and with the case still open, carefully turn it on for a brief moment. Only a few second are needed and the boot process don't need to complete.
If the fan is still noisy, it may be worn and need to be replaced.
Gently take OFF the fan, and ONLY the fan.
The exact procedure depends on the model of the fan, but, usualy, you only need a screwdriver, flat or philips head and some don't require any tool at all.
Take it to a computer shop and ask for a compatible replacement.
Put the replacement in place, connect it, close the case and power up.