MarzVP said:
I think it is a pocket of air in your heater core. My VP does the same thing if I have had the heater on before.
I've had this problem for a long time now! I think it maybe a small blockage in the heater core. I've found, whenever i switch the thermostat over to hot and have the heater running for 15-20 mins, for it to completely warm up, the noise comes, but then, i switch it straight over to cold and the noise has suddenly disappeared. I've had the cooling system completely flushed, over and over, but yet the problem remains. You don't have to worry about leaks in which the water is getting in, i'm sure this isn't the problem, i've spent a lot getting diagnostics done to test this theory, all have come back false. My best bet though would be a small (not complete) blockage in the heater core, and when the water flows through, it builds up soo much pressure that it sounds like it's gurgling under the dash. If you're really intent on fixing the situation, let me know how you go and if it is worth while, i may get it done too (i haven't been too game to rip half my dash out just yet!)
Take it easy and good luck!
Boojakz.