Tinnitus is the experience of sound in the head or ears that does not have an external source. It may take the form of hissing, buzzing whistling or other sounds. This condition is extremely common in mild forms, but about 5 percent of the population report that it affects their daily life negatively. It is estimated that some 50 million U.S. adults suffer from some form of tinnitus. In severe cases, it can cause insomnia, stress and difficulty concentrating and communicating.