The whistle in "A Song Written Easily" was recorded Yong Hoon of ONEWE. At first, he was not aware it was a ONEUS song. The recording involved two hours of whistling, during which he put in his best efforts to the extent that his mouth was cramping, and, in the end, the recording came out to his satisfaction and became a representative element of the song. He was not given any official credits for his contribution to the song.[1]