I just heard about this problem yesterday, guy at the office has it, it kinda comes and goes, he said his was caused by a muscle in his face that got hit and that muscle controls how much the ear hears, and that his was too good of hearing and he needs to get sound blocking plugs to quiet the noise.
I was wondering about tension, too, like when the vets grit their teeth at night and grind them to a pulp. When I grit my teeth then relax the ringing in my ears changes volume.