Odd noise

[ScoupeS12]

After I drive my S12...up to temperature, boot it a few times, (mostly sensible though) and then park it up, run for 30 sec's on idle then turn it off.

After about 2 mins it makes a load of bubbling noises sounds like water. Almost like an airlock.

But it doesn't use any water at all, and the temp is always spot on!

Any ideas? Places to start looking?