I've had a low-frequency hum, off and on, for over 20 years. I've never habituated to it. When it first started, I had numerous tests done including three MRI's over the years. No cause was ever found, and I had no hearing loss. My hearing is still great for my age (56). Sometimes the humming will go away for up to a year and then mysteriously return like it did last week. Mine may have started due to anxiety and depression. I've been on the antidepressant Lexapro for ten years, but it may be time to try something new.