Being extremely annoyed at the sound of someone eating or tapping their legs is an actual disorder. It's called Misophonia.
Misophonia, the "rage" some of us get when listening to certain sounds like people eating or breathing, is a disorder. The world misophonia literally means “hatred of sound” and is a form a decreased sound tolerance.
The term is interchangeable with the term Selective Sound Sensitivity and is targeted at specific sounds that hold negative meaning to the person whether loud or soft. The disorder isn’t entirely understood, but it might originate in the high central nervous system structure.
It is thought to be a result of an abnormally strong connection between the autonomic and limbic systems. People with misophonia tend to be annoyed or even enraged by specific sounds like people biting their nails or tapping their legs. People with it may also be diagnosed with mood disorders. Symptoms usually begin right around puberty.
It is speculated that the disorder is genetic. People use various coping methods. Most people simply leave the area, so that they don’t even have to listen to it. Others use headphones and music to drown out the sound. Some people actually feel the impulse to mimic what they are hearing. In extreme cases, the person distances themselves from socializing all together.