Gurgling Sound, Gurgling Sound coming from under my hood. Options

[5150EVH]

I recently noticed that when I come to a stop, I hear a gurgling sound coming from behind my firewall. I only hear this sound hear and there. I noticed too that when I turn my heat on the sound goes away....however, I smell a whiff of antifreeze when I first turn on the heat. I have been in the automotive business for 4 years (manager at Autozone and with CarQuest) and know that usually when there is overheating or this type of symptom and you turn your heater on and the problem goes away...its usually the thermostat. I was just wondering if anyone else has had this problem or if anyone could please give me some advice. I would greatly appreciate it. Thank you.

[5150EVH]

My problem usually starts after a few minutes of driving...if I turn on my heater, the sound goes away. I turned my heat off and drove for about 4-5 minutes and the sound was back and I smelt antifreeze as soon as I stopped. I popped my hood and antifreeze was coming out of my overflow tank. The carpet in my car is fine...I replaced my heater core last January (2007) and the floor has been dry since. The gurgling sound though does sound like its behind the dash in the middle which sounds like the heater core but when I pop the hood, the sound is right there near the front of the engine.