Could it be the Fuel Burning Heater shutting down? Not all facelift TD4s had them fitted, but if it has a heated front screen, it should have the FBH too. It sounds like a quiet fan along with a kettling sound, if it's been working hard.
If you have the FBH, there'll be a fuse in space 5 in the triangle fuse box and a silver (flue) pipe visible from under the LH front bumper corner. If you don't have the FBH, fuse 5 will be empty and there'll be a round hole in the bumper corner panel where the flue would poke out.