가슴이 콩닥콩닥 뛰다 · 콩닥거리다
thump-thump (heart fluttering)
Example
좋아하는 사람을 보니까 가슴이 콩닥콩닥 뛰어요.
Seeing the person I like makes my heart go thump-thump.
Word parts
Also the sound of grain being pounded in a mortar; today it's mostly used for a heart beating fast.
Good to know
The verb is 콩닥거리다. It sounds lighter and cuter than 두근두근, often for a crush.