Due to the high speed "24V DC Cooling Fan" have a greater noise than the low speed fan, so you should try and use low-speed fans as possible. And a large size, low speed fan, generally, quieter than the small size, high speed of the fan at the same air volume delivery. "24V DC Cooling Fan" temperature within a system, inversely proportional to the temperature required for cooling air flow and allowed. Allowable temperature rise slightly increased, to a significant reduction in the required amount of wind. Therefore, if "24V DC Cooling Fan" impose the permitted limit temperature rise slightly relax some, will reduce the required amount of wind noise can also be reduced.