Tinnitus caused from exposure to noise

By far the great majority of tinnitus sufferers have somehow damaged the delicate structures of the inner ear, resulting in damage to the cochlea, the cochlear nerve, the ascending auditory pathway, or the auditory cortex.
When the cochlear hair cells are injured by exposure to noise or head trauma, they discharge repetitively, thereby stimulating the nerve fibers to discharge synchronously in a way that the central auditory system cannot discriminate from actual sound.
This can be likened to a switch being left in the on position. i.e. Those hairs that are damaged are permanently in the "on" position, resulting in a continuous stream of perceived sounds being generated.
Tinnitus caused from sinus problems

Sinusitis and allergy sufferers also frequently suffer from tinnitus. This tinnitus "type" is caused from blocked Eustachian tubes, which results in a pressure buildup in the inner ear.
This condition is normally caused by prolonged taking of anti-histamines and antibiotics which can cause a thickening of the mucous in the inner ear.
Excess inner ear mucous would normally simply drain out through the Eustachian tubes but is prevented from doing so because it has become too thick to drain out of the very narrow Eustachian tubes.