Onomatopoeia refers to the formation of words, which (when spoken aloud) imitate the sounds related with the things/actions they refer to.
Beep: Short, high-pitched sound.
Flutter: Wave, fly, flap, flicker, etc.
They had no idea from where the beep sound was coming.
The beep of the car horn woke him up.
The dove fluttered its wings in pain.
The clothes fluttered in the breeze.