Discovery with water gurgling noise

Hi again. Fourth question of the day! My 2003 disco auto diesel has a loud water gurgling sound which seems to come from behind the dash. My mechanic (years of experience with Land Rover) has not come across it before and is baffled. It started months ago but seems to have got louder and more frequent recently. Its extremely annoying but I don't really want to go to the expense of putting it into a Land Rover dealership to find out what the problem is. Any ideas please? Thanks. Paul

Sometimes the clutch release bearing sounds like water.

Airlock in the heater matrix/coling system?

Check coolant levels, try taking a pipe off and flushing it through