Thanks to 4Eirlys for the prompt!
John was sort of proud that he knew things about Sherlock that no one else had any clue about. Sometimes he recited poetry in the shower. Sometimes he wandered off to stargaze because "it's nice to look at something I don't understand." Sometimes he drew whimsical spirals and patterns on his sheet music. John wouldn't ever share these artistic qualities; it was a side of Sherlock that he knew to be too intimate for anyone else to wrap their minds around. Besides, the bits of writing, astronomy, and drawings scattered throughout his day were simple pleasures he didn't want to lose.
There was one "artistic" quirk, though, that he could do without.
It was normal for Sherlock to hum softly to himself, especially during the construction of case maps. He would press tack and string to the wall with a decisive note, his voice flitting around with each connection. Pictures, ticket stubs, government documents—they all came together in a strange but coherent symphony.
It wasn't this John minded. It was when the humming stopped.
John lingered under his room's doorframe, trying to discern the screeches coming from the living room. Had a crow caught a wing in their window sill? Was a fox being tortured in the trash compactor?
Was Sherlock Holmes…singing?
The doctor crept around the edges of the wall, afraid that his presence would cease the sound before he caught a glimpse. He rounded the corner and watched as Sherlock distanced himself from the wall, approached it, then stepped away again. It feels like poetry in motion. His voice was deep, focusing on each syllable with care, and the volume was low enough that it shouldn't have been offensive but…goodness, it just didn't work.
Moon keep shining and I'll keep smiling.
Sherlock buried another tack into the wall and painted a red X over several pictures. John tried to suppress a smile. It was awful. Is distance really all that far? The blue robe fluttered as Sherlock again moved around the room; the different perspectives mustn't have been enough, though, and he gave a quick spin to readjust his sensors. It wasn't until he'd completed the 360 degrees that his voice and body froze.
"Lovely, that."
Sherlock spun back around. "Never a word. To anyone."
John lost all control over his grin.
"I mean it, John!"
The doctor moved his body behind the couch, suddenly considering the very real possibility of Sherlock charging at him. "How else am I supposed to explain my bleeding eardrums?"
Sherlock's face flushed. "I wasn't going for aesthetic quality," he murmured. "I didn't think you were home. It helps me to think."
"You mean you do this often? No wonder our neighbors hate us."
"Well if you're so good at it, show me!"
John frowned. "What?"
"Singing. Show me how to sing."
"I didn't say I was any good. I just don't sound like a dying owl."
"I don't believe you." Sherlock crossed his arms and flung himself on the couch. "You've no right to complain if you won't bother to show me a thing or two on the matter."
John sighed and began a quiet rendition of an Italian love song Sherlock had never heard.
The detective picked himself up after the second verse and stormed towards his bedroom.
"What, where are you going?"
"You're just a showoff," Sherlock said, and slammed the door.