The harder your cpu is working, the more heat it produces, so the faster (and nosier), the fans get to remove that heat.
Your cheapest options are to replace the fans with something better/quieter, or add extra fans to improver airflow.
Getting it right isnt easy, too powerful an input fan in the wrong place may disrupt the airflow going through an output fan, and make things worse!!
My system came with two Phantek fans, a 140mm (input), which makes the quietest of hissing sounds at full rpm, and a 200mm (input), which is a bit noisier, despite having a lower rpm. To this I have a watercooler that has just been fitted with a new pair of Scythe Gentle Typhoon 2250rpm PWM fans (output).
With the 200mm on minimum rpm, the GTs run...