Fan noise produced by the cooling fans for the radiator, alternator, and others, become louder in proportion to their rpm.
The fan noise can be produced by fan blades cutting through the air, or by air turbulences created by components located behind the fan. Fan noise can be reduced by altering the diameter, quantity, shape or angle of blades, as well as by using variable-pitch fans or modifying the fan shroud configuration.
In addition, it can be reduced by using a three-stage temperature-controlled fan coupling or an electric fan which keeps fan rotation to a minimum.