I am experiencing the same when I focus on hearing when there is no sound/noise (not an intense sound, but definitely a sense impression). Either both of us have bad ears or it's normal (it may be the sound of the blood floating in your brain or something). Nevertheless, if I don't keep my attention on it, it's not there.
I never did meditation on sound, but one option would be to chose another primary meditation object. There are enough of those. If you don't want to do breath meditation, you could use your body sensations as an object. There is a method called part by part sweeping (Link to a guided meditation by Ayya Khema), where you systematically notice sensations/feelings in your whole body. It has the advantage that you become more aware of emotions and reactions, since you are focused on all kinds of (changing) feelings and not i.e. only the breath going in and out. You can get quite concentrated with it, too.
If you want to stay with the sound as your object, you should notice the "hearing" and also your reaction to it ("worry", "dislike" ...).