You first need to identify which fan is making the noise.
Open the case and temporarily (two seconds) stop one fan by holding the center of the rotating part. Do that to all the fans untill you find out which one is the one with the most noise.
Then, you can work on possibly reducing its noise by reducing its RPM, replacing it with a larger (lower RPM) fan, replacing the whole cooler to one with a quieter fan, etc.