Heater Core!!! I tried quite a few posts on different forums with the same damn problem and no-one had a sure answer for it. But after a lot of fiddling i've come to the conclusion that the problem is a small block in the heater core. Even though the heater runs fine and is hot as, as soon as it warms up for a good 10-15 mins, you'll start to hear the gurgle every so often, flip the thermo straight over to 'cool' and hey presto, noise is gone. Since it costs an arm and a leg to replace the heater core, i've learned to just put up with the noise because it's not effecting the heat and there's no water or such pissing out on my feet! Oh...and its not the heater tap either, had mine replaced and the whole system pressure tested too many times and its always come back fine. It's up to you i suppose, go to a mechanic (mind you they'll still have no idea), tell them to check the heater core, and if there is a block, you can replace if you like, just have fun with the hefty bill at the end :yeah:
Cheers,
Boojakz