How Do I Bleed Air From Cooling System?

Hello All

The car is a 1990 Chevrolet Beretta with the 3.1 L V6 engine. I can hear water gurgling through the heater core (or at least somewhere behind the dash) when I start the car and the engine is cold.

I've tried adding coolant to the radiator several different ways and still I get the gurgling sound, which I am assuming means there is air in the cooling system. There is a small plug on the top of the thermostat housing. That's the only one I can find.

Anyway, can someone give me directions on the best way to bleed the air out of the cooling system. Or if you hae a different theory as to what the gurgling sound indicates, please pass that along, too.

Thanks in advance for any ideas on this.