Is it? I have experienced both these things:
1. A high-pitched hum, that is always there, if I choose to listen to it in a quiet place. (It's quiet enough now in my kitchen at 08:05; the only real sound I can hear is a collared dove every now and then.) If I really concentrate on it it can turn into a sound almost like crashing waves.
2. A ringing sound that is loud enough to interfere with hearing real sounds. I only have this a few times a year, often brought on by loud noises.
I had assumed that 1 is a standard feature of human hearing, while 2 is a medical condition called tinnitus. However, although lots of people have described 1, I'm not sure that everyone experiences it. It would be useful to have different names for these two things, even if they are to be described as two types of tinnitus.