I've had similar experiences before my real onset of tinnitus.
As a child, I always heard "something" in dead silence. It wasn't tonal, but more of a hiss. It would be very prominent right after a car ride when the car has stopped and there's just pure silence. I've discussed this phenomenon with many of my friends (who do not have tinnitus) and most have heard the same.
So I think almost everyone can hear a phantom sound in silence, but I guess shit really hits the fan when that becomes full blown
TINNITUS™.