If it has been raining outside then you may have a leak in the roof. If they are vented through the roof they may be leaking rain water around or through the duct. If these are new units then the tops of the units in the attic need to be covered with insulation as hot moist air is hitting the cold metal of the vent fans causing condensation. If they are not new and have been in use for some time and this problem has just started then I don't know what to tell you other than to check the insulation above the vent fans.