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bun-bun ぶんぶん the sound of humming, whirring, or high-pitched buzzing
chapu-chapu ちゃぷちゃぷ the sound of something being immerse in liquid; splish-splashing sound, such as feet stomping in puddles
hyuu ひゅう the sound of wind blowing; stronger wind can be expressed with hyuu-hyuu, byuu-byuu, pyuu-pyuu
nikkori にっこり also nikori and niko-niko, this expresses a happy smile or broad grin
pocha-pocha ぽちゃぽちゃ the sound of water moving about and splashing
shito-shito しとしと the sound of rain falling softly