thanks for getting back. For anyone else suffering this issue:
Low coolant levels seem to cause air bubbles to develope in the system, and make it sound like water sloshing/squirting sound under the dash (heater core). As it turns out, my car (80K miles) was leaking a good amount from the water pump. I replaced it, refilled the system, and still had the issue. Did some more driving, made sure the coolant was full, and drove it more. Eventually, it went away (2 days or so). Maybe there is a better way to bleed the system, but eventually it worked itself out.
