A short story from the real world. When I was in Bangkok for my first stem cell treatment, each day daily treatment session would start at around 10 o'clock with a briefing between myself, my assigned physician, and the medical director. Usually fairly informal - more of a chit chat and a bit of coffee, really...! For one of those briefings, a senior physician happened to walk by as we were having our discussion. He joined in for 15-20 minutes just to hear a little bit about my case, background, and reason for developing tinnitus. I mentioned that I had been using CIPRO (eardrops) in my left ear a couple of times in the past years (ie. for a week's treatment on a couple of occasions). And consequently, I suspected this to be the culprit for developing tinnitus in that ear. And so we had discussion about ototoxicity and how it differs depending on medication, dosage, and delivery method (oral vs. IV). The senior physician specifically mentioned that ototoxicity sometimes does not show up until weeks/months after cessation of treatment. It also means that the damage is not always done in "one go", but may be "on-going" for a while. This could be the case with you, therefore (based on what I read).
On another occassion, I was getting involved in off-label AM101 treatment at a clinic here in Germany. I was seen by one of the doctors at the Acquaklinik here in Leipzig, who is fairly "aggressive" in attempting to cure tinnitus, and will do "avantgarde" approaches to prevent/cure tinnitus. One example - he mentioned - was preventing tinnitus to occur in a female patient who was about to undergo chemotherapy. By delivering steriod intratympanic injections before the chemotherapy began (and also during treatment, I think) he prevented hearing loss and tinnitus from occuring. He also did mention that he sometimes will use steriod intratympanic injections several months after tinnitus has developed - even though it may not be effective.
Ototoxicity is therefore something that needs to monitored and managed when it is suspected. It is not enough to simply conclude that it has occured. It is also possible to put on high doses of certain antioxidants (IV) - when can help prevent damage from progressing. It seems your doctors do not know this kind of specific information (I am not surprised). And therefore did not consider it. It is also not enough to administer standard audiogram measurements to track hearing loss from ototoxicity. Hearing loss from ototoxic medication will usually affect the very high frequencies (> 8 kHz) to begin with. Having a standard audiogram done - which are by far the most common - proves essentially nothing therefore.
Sounds like reactive tinnitus ie. tinnitus which amplifies itself in certain sound environments (eg. driving a car or being next to a computer fan). It is quite common.
When using earplugs, it is important to let the foam expand inside the ear canal while keeping the plugs pressed in. If you simply put them in and let them expand on their own, you will not get the full benefit of the protection they offer. Far from. This is probably the most important step - and probably the one people most often neglect. Or are unaware of...