I can answer the first part of your question, what is sound? When you speak or a speaker vibrates or a loud motorcycle drives by, you perceive a sound. What happens is the air near the source of the sound gets compressed and then expands again only to compress the air next to it. This compression wave travels away from the source in a sphere like after a rock in thrown in a pond. After a short time this compression wave hits your ear drum and little bones, and nerves in your inner ear change this into a signal that your brain learns to interpret as sound. Sound can travel through other materials as well, not just air. You can hear noises under water, or by tapping on a metal pipe or through a string if you have ever attached two paper cups to a tight string.

As for the other part of your question you would have to ask a psychologist or evolutionary biologist. I would say loud noises are disturbing because they may be unexpected. If your blood pressure increases, it is likely to make more blood flow to your brain making you more alert so you can react to any danger.