Bata bata

Bata bata is onomatopoeia for running sound, especially used when someone is in a hurry.

It could also used to describe the sound when things are flapping.
(e.g. flag is flapping when the wind is blowing etc...)

It could also used when people are swimming and they are kicking their feet.

i.e)
Jp: kanojo ha kyou ha isogashisouni BATA BATA siteiru ne.
　かのじょは きょうは いそがしそうに バタバタしているね。
En: She is running around noisily today. (she isn't actually running)

Jp: sentaku mono ga kaze de BATA BATA to yurete iru.
　せんたくものがかぜでばたばたとゆれている
En: Laundry is heavily swinging by wind.