Sounds like your heater core or the associated lines to it are\is leaking. Maybe someone knows how better to check and or replace it. I had a heater core go out on a cavalier, and would hear a splashing noise when accelerating. That was air bubbles in the heater core. If your heat stopped working, I would suspect the core or the inlet / outlet lines. If you see coolant around the firewall, check the lines running to the heater core. Hopefully the core itself isn't bad.
Replacing the heater core looks to be a huge pain. Looks like you have to discharge the ac, disconnect the blower motor, remove the HVAC housing after removing the instrument panel cluster, steering column covers, instrument panel trim panels, knee bolster, glove box, cowl support beam and floor heating ducts.