Hi Aly
I'm glad you've been thoroughly checked out, it at least rules out any nasties
Do you have any sinus problems at all - I ask as I did my Scuba training in open water & got into big trouble with my balance when I went down - everything just lost the plot & I couldn't tell what was up or down & felt I was being spun around - I didn't know then, but was later diagnosed with a deformity in my Sinuses that needed surgery & according to the ENT I shouldn't dive - my Tinnitus definitely got worse for a while after that, but eased down over time.
I don't ever remember NOT having Tinnitus & just thought it was normal, I had no idea until I read an article on it, probably in my 20s, that not everyone has it - thats how normal it was for me - but it did get worse after a minor neck injury at 19, that also kicked off migraine, hence why I was looking into it all & there does seem to be a link with the 2 for me - I've also read & been told, that its also linked to Fibromyalgia, which I was diagnosed with 9 yr ago, but believe I've had since I was small.
Mine fluctuates, both in sound & intensity, & can range from what you describe, to a high pitched whine that you would get with a test card back in the old days, to whooshing as if listening to a sea shell, or the tinkling of loads of tiny bells.
I've no idea if what you experience is worse or not, but I do know if I concentrate on it - which I did after reading your post, it gets MUCH bigger & can take over me & drive me nuts - IYSWIM, so I really do think learning to tune it out is the key - if you are not naturally that way inclined, then I understand CBT or Hypnosis may be of help - our minds are pretty powerful things & really can be the best tool for dealing with any of these things :)
I have a friend whose partner is an NHS CBT therapist & having spoken with him about it, it really is just what I call "mind over matter" & I'm lucky to be that way inclined naturally - but talking to him & others who've had CBT, its techniques to retrain your mind to be that way - it works for "tuning out" pain too.
The other thing that I've found can help & might be worth looking into is Chiropractic - thats if theres any possibility yours could be linked to neck pain/problems - my migraine & tinnitus definitely get worse if my neck is tight, sore - massage can help with this too.
The other thing that springs to mind is that you sound very anxious - this won't help you cope at all - lots of people are deficient in minerals, modern diet & less in the soil than they used to be means even healthy eaters can suffer, & some drugs leach some minerals from your body - I had a BP drug leach the Potassium from mine & I ended up in A&E as a result - turned out to be a panic attack cause by low Potassium.
Magnesium & Potassium are both important minerals for regulating your bodies response to stress, pain etc etc - Taking Spirulina & either a magnesium supplement or regular soaks in baths with 2 cups of bath grade Epsom Salts - foot bath will work if you've only a shower - keeping up your intake of these can help a lot
I hope you manage to find something that helps you cope soon
good luck