The HVAC Air box inside the cabin (above your feet and across the front fire wall) has a small rubber drain to dump the AC Evaporator condensation that happens when you run the AC. The condensate usually drains out under the cars passengers side and drips on the ground when the AC is running on a humid day. IF,, that condensate drain gets clogged (and they do) the condensate water stays inside the HVAC Air box and sloshes around. I also agree with the theory of water in the windshield COWL area. Check out those two problem areas.

Bill