Check that the buzzing isn't from a fault in the head unit---get a cheap speaker (even a PC speaker or one out of a busted clock radio or something) and hook it directly to one of the speaker outputs of the head unit. If that buzzes, it's probably something in the head unit.
Also, make sure that you didn't cross any of the speaker wires, as in left front negative got connected to the same speaker as right rear positive.
