And he heard a slushing sound, metal moving across an oiled surface.

If the cooling is very fast then the outer layers of liquid freeze before the inner liquid is cooled creating "slushing."

No school-children so far, but I can hear the first faint scrapes and slushing of householders beginning to clean their pavements, and the sound is strange and hard to recognise in the almost silent air, seeming as if it came from a long distance, a country sound in the wrong place.