I have it too.
Some say it can be started from exposure to constant noise. That would be from the nearly ten years pulling Alerts in the missile capsules. Quite noisy with the air conditioning systems, motor generator, and other constant whine of equipment.
Mine is only in the left ear. On hearing this a friend of mine asked if I drive with the window down (yes), he called it “truckers ear.” Even when it is very hot or very cold out, I always have the window cracked down just a bit, I just like to have a steady flow of fresh air.