If no cause can be found after various tests have been performed,
then it is most likely due to MUSCLE TENSION.
Please see a massage therapist - trusted chiropractor,
physiotherapist, aroma therapist etc.
See if there is tension in your body. Ask questions of the therapist.
Where is the tension?
What can I do about it etc?

Then ask your self: What am I doing during the day that could be
causing this?
Do you clamp a phone in your neck? - get a headset.
Are you sitting badly? - improve your posture.
Is your computer positioned correctly?

Is the monitor at eye level - no peering up or down.
Is the keyboard roughly at your waist level?
Is your chair comfortable? no slouching.

Do you drive a lot? see travel tips
Try some of the neck exercises at:
http://www.nismat.org/orthocor/programs/neck/neckex.html

3. A relaxation exercise by an internet ENT, Dr Murray Grossan:
http://www.tinnitusrelief.net/

I had a brief episode some years ago. I had become aware of muscular
tension by this time, so I got rid of it as soon as I could.
I had been a passenger in a a car on a long trip. After driving for 2
whole days I began to feel as if an iron band had grown around my
chest. When I bent down to pick something up off the floor the PT
kicked in.
My seating position had not been what it normally is when I am the
driver. I usually have a cushion in my back and sit up straight.
Because I was a passenger I thought this was not so important. But I
was wrong.
I put the cushion back and sat upright for the next week. The PT went.

Elly's Tinnitus Resources
http://eebee.net/