A transverse fan impeller (10) having blades (31) that extend longitudinally parallel to the rotational axis of the impeller. Each blade has an airfoil cross section, defined by a chord line (Ch) and a camber line (Ca), and is positioned in the impeller at a setting angle (.GAMMA.). The outermost edge (Eo) of each blade is located at a distance (Rmax) from the rotational axis (Ar, Ar'). The angular spacing (.SIGMA.) between blades in the fan is uniform. Among the blades of the impeller, the values of at least one of the parameters distance from outermost edge to rotational axis (Rmax), length of chord line (Ch), maximum deviation of camber line from chord line (Dmax) and setting angle of blades vary randomly from reference values of these parameters. The random parametric variations reduce the blade rate tonal noise produced by a fan having such an impeller as compared to a fan having an impeller with uniform blade parameters.