Leaf blowers immediately come to mind, jarring my sense of calm. The loud brrrr of the motor always ticks me off. The landscapers always do their work very early in the morning. So I've woken up more than once to that insufferable sound right outside my window. Motorcycles and lawnmowers give the same effect for me, and I detest when folks rev their engines wildly on the road. It's angry and aggressive enough to unnerve you.
Another crucially annoying sound is that of car alarms. I'm talking the most common alarm system that plays out a series of awful tones in succession. You know what I mean. I doubt anyone would disagree on this. At first I thought they were funny when I was a kid. I quickly grew out of that opinion after they woke me up in the middle of the night every time a cat jumped onto someone's car. Ugh...*massages temples*
Last but not least is just a personal peeve of mine. I hate it when people need to belch or yawn in the middle of their sentence, but they keep on talking instead. So their mouth gets all wide and you see their tonsils while they're trying to suck in air. They sound like they're drunk or sleepy. All of this weakening their words that are mashing together like auditory rotten bananas. I want to scream: "Just let it out already!" YMMV.
I think that the most annoying sound in the world would have to be girls screaming to talk to eachother. This gives me a headach, and sometimes I just want to slap them to shut them up. And no Jasmine this isn't just you and your friends. Melinda also does the same thing when she is around her friends.