My 2016 A3 2.0tdi stronic quattro has developed a cabin heating issue, the heater blows hot air for about 10 minutes then goes cold, the coolant level in the reservoir is fine and there's no leaks, the car isn't overheating, I can hear movement from behind the heater controls when I operate the directional buttons, I can feel both pipes leading through the firewall are hot to the touch when the car is up to temperature, is anyone able to shed some light on a cause and cure for this please ?