What is the meaning of "tump-tumping" in the extract below. I looked in many dictionaries but the expression is listed nowhere.
From the novel, Waterland:
Fairy-tale words; fairy-tale advice. […] So he would always set his eel traps at night. Not because eel traps cannot be set by day, but because the mystery of darkness appealed to him. And one night, in midsummer, in 1937, we went with him, Dick and I, to set traps near Stott’s Bridge. It was hot and windless. When the traps had been set we lay back on the river-bank. Dick was fourteen and I was ten. The pumps were tump-tumping, as they do, incessantly, so that you scarcely notice them, all over the Fens, and frogs were croaking in the ditches. Up above, the sky swarmed with stars which seemed to multiply as we looked at them. And as we lay, Dad said: ‘Do you know what the stars are? They are the silver dust of God’s blessing.