The RPM of the fans are adjusted according to the temperatures of the components. During a heavy load on the system the fan's RPM will increase as they are supposed to. However, sometimes the the air to to the components may be minimised due to dust and other obstacles. One solution may be to clean the computer, this may fix the problem. Or if possible, you can look for other more quiet fans to replace the old ones with. Or you can do like me, just use noise reduction headphones