I absolutely need background noise at all times. When I read, I've always got my iPod going (usually instrumental/classical music... lyrics might distract me). When I'm writing a paper or something for school, I've got iTunes open. When I go to bed, I've got a DVD on and it's set on repeat to play throughout the night.
I think part of my problem is paranoia. If it's dead silent and I hear an odd noise in the background, it catches my attention and then I wonder what caused it. Also, when I am at home, the sound of running water/pipes makes me tense up. I've experienced a couple of water leaks in the past, and so now every time an upstairs neighbor is flushing a toilet or running the water, I get paranoid and worried. Silly, but it happens. Any noise to help drown out the sound of the pipes helps calm my fears.