I'm not a physician or neurologist, but I read a lot about this when I started suffering from tinnitus. There are indeed multiple types.
- Hearing loss related: the tiny hairs in the inner ear that transform vibrations into chemo-electric signals get damaged and start sending the wrong signal.
- Cardiovascular: a pulsatile rumbling that varies with heart rate. It's the sound of the blood flowing through some artery near the inner ear. I remember reading that it was caused by loud arteries and that it could be detected with very sensitive microphones.
- TMJ/cervical/postural: I think this one was about motor system or pain signals leaking to the ear circuit, but I'm not sure.
- Timpani related: wax buildup, tensor muscle acting funny...
This rough classification only deals with the "input" part, though. It doesn't explain how these bad inputs reach conscious hearing, causing distress. As a general rule, over time extreme pain becomes just pain, torrid heat becomes just heat, blue and red can be seen as white depending on the lighting conditions, even hunger and thirst are modulated by the nervous system. Eventually one learns to cope with tinnitus, but it's kind of fishy that this isn't filtered upstream by the auditory system, before it reaches conscious hearing.