Tinitus.
When I was a teenager, the world could be blocked out easily with headphones and a high volume setting. This, understandably, made my ears ring constantly. I have since grown up, but still block the world out with headphones, to save having to listen to a confusing mass of languages, voices, accents, engines, piped music, noise from other people's headphones, or the wind through the trees, the buildings, or just hitting my ears at speed while I'm cycling. I have grown used to the headphones. If I have to walk anywhere without them, and without a convenient wall to check my bearings against, I tend to weave in a slightly drunken manner. When everyone else hears silence, I hear a faint high-pitched tone reminding me that, someday, I should turn the music down before I go deaf.
But every time I look at the volume control, or even go so far as to turn it down, I'm reminded by something irritatingly loud or just irritating nearby.
On top of this, I grew up in the Reading area. As mentioned above, trains, cars, aircraft. All day, all night, constant background noise, with Concorde twice a day, drowning everything else out with a noise like tearing silk in a firestorm, regular as clockwork until the very last flight, and a disturbing quiet ever after. Even after moving a few miles away, I swapped the cars for louder aircraft, a different train line, and frequent low-altitude helicopters. I have grown to recognise the thudding of an RAF Chinook, and distinguish it from the high-pitched rattle of civilian helicopters, and the dwelling drone of the police. When the sun is up, there are birds. Loud birds. Birds while I'm trying to sleep through the approacing dawn, while I'm trying to eat, trying to write. I'm used to constant noise. If all is silent apart from the ever-present ring of hearing damage and the familiar thud of my heartbeat, I feel a sense of unease. This Should Not Be. Something Has Gone Horribly Wrong (and not just my internet connection deciding to die the second I type that). As far as I'm concerned, the world should not be silent.
Even as I sit here, in the middle of the night, I could turn everything off. The loud whining hum of the computer I'm using while my regular one is being fixed, the TV set to a radio station, the hum of the monitor... I don't want to. Sure, it could do with some adjustment, as the computer's noises are boring into my skull painfully, but I like the noise. I welcome it. I crave it; just on my terms, not on those of the people around me talking loudly about petty social matters I have no understanding of or interest in.