Castiel slowed down on his bike when he recognized the familiar spot where he first heard the singing. He fully stopped and waited, holding his breath. As if on cue, the singing started up. It took a minute for the teen to recognize the song, but grinned when he recognized it. They did a great job at singing the emotions within the lyrics, though there was pauses that were probably for catching on their breath.
The teen hummed the song along with the singer, wishing he knew just who it was. Hopefully someone his age; they could start a band if so. If they were older, then just chatting would be fine.
But did he really want to know who the mysterious singing angel was?
The final "somebody to love" was belted out with raw passion and emotion, and that's when Castiel knew the answer to his own question. Yes, he wanted to know the person behind the voice. To thank them for keeping their window open when they sing, to maybe sing a long sometime. He barely even recognized the soft grin on his face; he felt his grin was like the wavy smile cartoon characters had when they were in love.
Not that he was in love; appreciating art and loving somebody were different things after all. He hopped back on his bike, making a silent promise to the secret singer to always be their secret audience.