Unless you find evidence of an external leak, Id suspect a head gasket failure between a bore and a coolant passageway. Hopefully it's the gasket and not the head or block.
I have just replaced the rad and water pump on my 300Tdi RRC, and had a little trouble bleeding all the air out afterwards - I refilled it cold through the header tank, rad bung and thermostat bung, and then ran it for five minutes or so to circulate the coolant, topping up the levels again. using the car the next day was fine, but the day after, the heater matrix produced occasional gurgling noises, particularly when the engine rpm was changing significantly. The header tank level was fine, but the thermostat housing was dry, so I topped it up through its bung again. This has needed doing twice more, but the system is now purged. The engine temperature has been absolutely normal throughout, so the gurgling is more of an inconvenience and annoyance than a threat - it doesn't take much air to do it.
