Requiem
Sherlock was constantly frustrated and in awe of Mozart's Requiem in D Minor. As a musician and an egomaniac, he adored it. Its layered chords made him feel like royalty as he strutted through stairwell of Baker Street. He tended to play it on repeat while he was thinking, and after a while it began to drive John absolutely mad. He taunted Sherlock and told him to stop pining over the damn opera, telling him his recent obsession with the aristocracy reminded him of Moriarty. It creeped him out, nonetheless. Sherlock didn't' care. Each and every note of that piece was like crystals shining beneath his footsteps, magical, ethereal. As a violinist, it was torture. There was no way to capture the magic of Mozart's Requiem on a single violin. He had tried composing his own arrangement but when it came down to it, the notes sounded boring without the glorious foreground of a choir. John had reminded him he was a violinist and a detective, not an operetta. To which Sherlock replied that an operetta was a genre of opera, not a bloody singer. He lost sleep over that piece. However, he never would admit it was his favorite.
Prodigy
"As a child," Mycroft began,
"Sherlock was very determined. Determined to be the best at nearly everything. He wanted to be a prodigy. He wasn't, though, obviously. The trickiest bit was the violin. The moment he picked up the violin it was a new competition, a marvelous way for him to prove that mummy loved him best. I watched him scurry to lessons, practice for hours into the night, and get virtually nowhere."
"He's quite good now, though." John said,
"So he must not be as bad as you reckon."
"No, no. He studied profusely, John, of course he's quite good. You should have seen him in secondary school; he neglected nearly everything in order to play. He wanted so badly to enter all the concerto competitions, to prove he was the best. But he knew he wasn't. He always deduced who would win before even considering his own chances. Honestly, it was quite sad." He sighed,
"That's enough sentiment for now, don't you think?"
The Affair
Oftentimes, John would wake at night to find Sherlock had disappeared from his side of the bed. Each time, he rolled his eyes and mumbled, "Again?" Before wandering into the sitting room to discover whatever the hell he was about to discover. Sometimes he was simply staring out the window. Other times he had completely gone, scaring John half to death. Once, it was to buy an ancient Turkish sword from the black market in the downstairs lobby. This time, though, John awoke to the melodies of his violin. It wasn't too unusual, save for the fact it was three thirty in the morning and Sherlock had surely gone to bed hours beforehand. The sounds of the strings echoed eerily through the night and John recognized the tune: deep, romantic, and vibrating. Sherlock claimed he played in order to think, but John knew it was always more than that. It was as if he was engaged in an endless love affair, passionately playing into the dark hours of night. No one could interrupt him or lay a finger on the instrument. He cared more for his violin than he did for himself. And as John wrapped his tired arms around Sherlock's waist from behind, he knew it was odd moments like these that showed his humanity. He wouldn't tell him to shut up this time. He would simply lean his head against his back as his shoulders moved with the sound, and listen.
Horror
There are some terrible noises that can come from a violin. Absolutely terrible.
For example, if instead of playing between the bridge and the fingerboard, one plays between the bridge and tailpiece, especially on the wound part of the string, it will emit a terrible screeching noise. Sherlock particularly enjoyed this noise for getting attention. He would cease playing and simply screech away until John noticed him. It worked every time.
Two notes that go together particularly atrociously are the E and Dsharp pairing on the A and E strings, played by nudging the third finger up a little too high and droning on both strings at once. It has a unique ring to it that bothers the ears. It notably irked Mycroft. In fact, it had bothered him since he was ten years old and Sherlock came into the kitchen, blaring the chord into his ears. It was quite funny.
Many people think high notes, above the natural ledgers of the scale, are wailing, awful, and not to see the light of day. This is untrue, of course, unless they are played badly. When one is capable of playing them well, there sometimes is nothing better than torturing others with the sound of a badly executed seventh position note. There was sheer horror on Mrs. Hudson's face when Sherlock sustained a disgusting, out of tune note at the highest octave.
Ricochet is a type of bow technique that when done properly creates a bouncing, spooky sort of emotion that adds to the character of music. It can also sound very annoying, and when done absolutely terribly, sounds like a bow might be breaking. Lestrade may have threatened once to break Sherlock's bow himself if he were to continue.
In sum, this is only a summary of noises that can come from a violin that are not so melodic and can be torturous to their listener. Which, of course, as a result, have been used within the walls of 221B Baker Street.