Subjective tinnitus is always a symptom in the brain, but it is almost always caused by a malfunctioning of the auditory system and the brain trying to compensate. Certain disorders such as Williams syndrome, autism, Lyme's disease, syphilis, depression, Schizophrenia, etc. can cause tinnitus in the brain without a whole lot of auditory damage, but this is not common. It's typically a fairly safe assumption that tinnitus is caused by an irregular auditory input, and the brain undergoing neural plasticity in an effort to salvage the hearing input.