First off to purge the radiator(not sure on the gen III's, but this is on the gen II's for sure.) you just purge air by using the bleeder screw on the thermostat waterneck housing. Secondly, I thought that our cars came with a "catch tray"(not sure of technical name)of sorts in the dash area that have a drain tube in them. if this tube is plugged the tray could have gotten water in it and now it sloshes around. These were originally intended to catch any leaks from the heater core so that they dont explode or leak coolant onto your cars floor. Thay is my guess as to what you may be hearing as well. This tray should be located under your heater core. Hope this helps...
