The problem is that the best you can do with the design of a fan is to eliminate the noise of its motor. But, the noise from the air movement remains. After all, a fan is supposed to move air. You can completely eliminate the noise of the motor by placing the fan in another room and using a pipe to bring the air to the PC. This of course is a ridiculous idea since the resistance of the pipe will dramatically reduce the air flow. But, if you can imagine an ideal imaginary pipe with no resistance! The air that moves out of the pipe and into the PC will go through obstacles like the grid or the components inside the case. This movement of air through obstacles creates noise. You cannot eliminate this kind of noise by changing the fan.