My father and my son both can't sleep without a fan. Ceiling fans don't count because they're too quiet.
When we moved from the middle of town (a block from the railroad tracks, no less) out here to the country, my girlfriend had trouble sleeping because it was too quiet. Then she had trouble sleeping because of my snoring. You just can't please some people :)
I can't sleep if I can hear voices. Other noises generally don't bother me, but my brain tries to listen to whatever is being said.