A cooling fan includes a housing, a cover on the housing and an impeller received in a space between the housing and the cover. The housing includes a bottom wall and a sidewall projecting upwardly from an outer periphery of the bottom wall. The sidewall defines an air outlet therein. The impeller includes a hub and a plurality of blades extending radially out from the hub. A porous layer extends into the space between the housing and the cover from the sidewall at a location adjacent to the air outlet.