Tinnitus can arise anywhere along the auditory pathway, from the outer ear through the middle and inner ear to the brain's auditory cortex, where it's thought to be encoded (in a sense, imprinted). One of the most common causes of tinnitus is damage to the hair cells in the cochlea (see "Auditory pathways and tinnitus"). These cells help transform sound waves into nerve signals. If the auditory pathways or circuits in the brain don't receive the signals they're expecting from the cochlea, the brain in effect "turns up the gain" on those pathways in an effort to detect the signal — in much the same way that you turn up the volume on a car radio when you're trying to find a station's signal. The resulting electrical noise takes the form of tinnitus — a sound that is high-pitched if hearing loss is in the high-frequency range and low-pitched if it's in the low-frequency range. This kind of tinnitus resembles phantom limb pain in an amputee — the brain is producing abnormal nerve signals to compensate for missing input.
There seems to be a two-way-street relationship between tinnitus and sleep problems. The symptoms of tinnitus can interfere with sleeping well—and poor sleep can make tinnitus more aggravating and difficult to manage effectively. In the same study that found a majority of people with tinnitus had a sleep disorder, the scientists also found that the presence of sleep disorders made tinnitus more disruptive.

Pulsatile tinnitus: This problem usually is related to blood flow, either through normal or abnormal blood vessels near the ear. Causes of pulsatile tinnitus include pregnancy, anemia (lack of blood cells), overactive thyroid, or tumors involving blood vessels near the ear. Pulsatile tinnitus also can be caused by a condition known as benign intracranial hypertension (an increase in the pressure of the fluid surrounding the brain).
Schecklmann et al (2014) suggested that tinnitus is associated with alterations in motor cortex excitability, by pooling several studies, and reported that there are differences in intracortical inhibition, intra-cortical facilitation, and cortical silent period. We doubt that this means that motor cortex excitability causes tinnitus, but rather we suspect that these findings reflect features of brain organization that may predispose certain persons to develop tinnitus over someone else.
Objects or insects in the ear can be placed in the ear by patients themselves, or an insect crawling in the ear. Ear wax can also cause ear problems if Q-tips are overused to clean the ears. Symptoms of an object in the ear are inflammation and sensitivity, redness, or discharge of pus or blood. When to seek medical care for an object or insect in the ear is included in the article information.
The treatment involves implanting a small electrode into a person’s neck near the vagus nerve. The patient then listens to specific tones that are paired with small electric pulses sent to the vagus nerve. This vagus nerve stimulation, coupled with the sound-based stimulation of the auditory cortex, can “turn down” the patient’s tinnitus. Though, Kilgard adds, “It’s not 100 percent yet.”

It is very well accepted that tinnitus often is "centralized" -- while it is usually initiated with an inner ear event, persistent tinnitus is associated with changes in central auditory processing (Adjamian et al, 2009). Sometimes this idea is used to put forth a "therapeutic nihilism" -- suggesting that fixing the "cause" -- i.e. inner ear disorder -- will not make the tinnitus go away.   This to us seems overly simplistic -- while it is clear that the central nervous system participates in perception of sounds, and thus must be a participant in the "tinnitus" process, we think that it is implausible that in most cases that there is not an underlying "driver" for persistent tinnitus.
To answer your question about NAC (N-acetyl-L-cysteine), I’ve seen little evidence suggesting that is effective for tinnitus. Instead – though the research is very limited – multiple anecdotal reports describe success with the herb ginkgo biloba. Try taking two tablets of standardized extract of ginkgo three times a day with meals (no more than a total dose of 240 mg a day). Ginkgo may work by increasing blood circulation to the head and neck. Give it at least a two-month trial. You might also explore cranial therapy, a gentle manipulative technique performed by osteopathic physicians. This approach seems to take the pressure or irritation off the auditory nerves. If high blood pressure is responsible for your tinnitus, try to get that under control through diet, exercise, and weight loss or medication if necessary.
Loud noise exposure: Being exposed to occupational loud noise on a regular basis from heavy equipment, chain saws or firearms is a common cause of tinnitus. However, even if you don’t work in a noisy environment, you can still suffer the effects of noise exposure by listening to loud music through headphones, attending live music performances frequently and engaging in noisy hobbies.