@monkey124
ENTs also don't know. Audiograms are fine. One (of the 4 that I saw in total) ENT said it was probably something in my neck... but that is absolutely not what it is.
I did have an "accident" with earbuds 4 weeks prior to the low rumble kicking in, so I am attributing it to that, although usually - tinnitus from acoustic trauma is pretty instant (from what I've come to know).
So - currently I don't hear the low rumbling one (it sounds exactly as you describe it - car / truck idling outside/inside my head) - I 'only' have very high pitched static hiss in both ears for the moment (right "bad" ear side usually a bit worse).
Another thing is that there is definite constant 'pressure' in my bad ear.
Also - like many other people do: wear earplugs in the car. I am not able to do that. If I drive my car with earplugs in the bad ear, the tinnitus is going extremely loud. Unbearable.
Currently waiting for my shipment of NAD+ and Resveratrol - don't know what else to do tbh.
I know for a fact that the low rumbling is not gone, it's just that I keep my brain busy 24/7... if I don't (> sleep on my right side, put my bad ear on the pillow and therefor shielding it from my masking sound), the rumble will definitely be back, as I said before > tried and tested 2 weeks ago.
I had the same feeling as you describe... even if, some days, the rumble wasn't really loud. As soon as I found myself in a quiet room (trying to get to sleep or something), my brain would start tuning in and make it louder and louder ...
So - again - I have low rumbling masking on Day and Night... In the day I add music to it and in the night I have some random nature sounds (I use asoftmurmur dot com - it is free and they have good quality sounds you can mix)
(Sorry for jumping from one thing to another - it's late where I am and I am tired)