coolant leak

[thomas12]

My Passat, V6, is loosing coolant. I noticed that it happens only when the car cools down.
I park the car and there is no coolant under the car for a while but when it cools down looks like it's leaking somewhere between the wheels. Also, when I start the car in the morning (after long park) I hear the water bubbling behind the dash.
The noise disappears after couple of minutes. It sounds like the coolant is filling the empty hose.
Do you have any advise where is the leak and what is the problem?
Thanks,

[veedubmechanic]

water pumps are the most common area to leak. there is a coolant flange at the back of the engine.