Justin Bieber Leaks Tunes In Men's Room: Star Caught Singing — While Peeing!

Justin Bieber surprised a gentlemen in a men's room in New York City recently, when he walked in alone — rapping out loud!

What's more, sources tell Straight Shuter that the singer even gets louder as he pees!

"He was singing and rapping as he peed and he didn't care who was listening," a source said of Bieber's private bathroom concert. "Justin just wanted to pee and he wanted to sing — and that's exactly what he did."