とんとん (light tapping/knocking)

A 擬音語 (sound mimetic) for light, rhythmic tapping or knocking — a finger on a table, a hand at a door, drumming fingers, light footsteps. The reduplication captures the repeated rhythm of the sound. Also extended metaphorically to describe smooth, regular progression as in トントン拍子 ('one-two-three success'). Sometimes written in katakana (トントン) for emphasis.