Kpop Star IU finally explained the scream in her title song 'You and I.'

Lee Min Soo, the songwriter who wrote 'You and I', revealed that the scream was inserted in the the later parts of the song intentionally and that it was IU's own voice. 

He said that the decision was made after a discussion with everyone involved with the recording such as the producer and lyricist. He also confessed that he was disappointed that it didn't get as much attention as he expected.

"The reason the scream was added was due to the theme of the song," he explained, "'You and I' is fantasy story about time travel as a teenage girl visits her future. The first verse is about the past, second about the present and the third symbolizes the future. First verse is meant to sound wistful, second to sound frustrated and the third verse is meant to sound  a little desperate." He explained that the scream was inserted into the second verse to express the frustration.