The worst sound is the sound of a truck or bus's air brakes. From a distance it isn't that bad but if you're right there next to it it's the worst noise in the world.
Well, I can't stand the noise is when some one scrapes thier fingernails and a chalkboard, mosquitoes buzzing over my head when I'm trying to sleep, and when I hear the buzzing noise from a TV.
Back when I went to elementary school, there was this alarm that played whenever the fire alarm was activated. I always closed my ears when the sound played. I still close my ears when I hear that alarm at high school.
@Chata722: I so hate it when an insect is buzzing. Makes me wanna get a sledgehammer and smack it a thousand times on the floor.