It is a barometer by which to gauge your bird's health. In this same way your bird may be trying express a want or need.
Stress can cause the bird, if it persists to disturb, to pluck until it bleeds.
The way to soothe the bird is to remove the loud noises and create a more serene environment for the bird. Mites should be treated by the veterinarian. Biting can be a sign that your bird feels neglected. The bird is expressing to that it feels left out of your life. If you play with the bird more, in a couple of days it will be back to normal.