usually the humming noise comes from a dug wheel well you should check if the tires are perfectly balanced...........or the noise comes from the engine section, or the propeller shaft, or if it comes very nearer to ur compartment the gear box for front side and differential for rear side........well with out check in it there could be no answer for it, you can basically first check from where the noise is coming front or back?? coming from left or right?? hoping some wheel bearing also makes noise if they r worn so as i said diagnostic is necessary...do it your self or Ford Motors Always there!!!