Others will need to practice patiently, but it will come. At first, the tone will sound far away, barely audible, like static, like the white noise of a television channel with no station on it. You may hear it first within one ear more than the other. When you first become aware of the sound, you may feel that the pressure shifted in your head, like you've just yawned or stepped out of a swimming pool. Try to pinpoint the sound.
Most people come to identify it as coming from the base of the skull.
Practice refining the sound. Learn to tune into it more clearly. The static-like quality will change when you really listen to a more pure, resonant tone. As you learn to do this, let the sound permeate your entire awareness, ringing quietly through your body and your mind. Breathe in this silent sound.