It may not make a lot of sense but it is a thing that happens often. You hear something but there is nothing evident that could make any sound. At least not in the narrow viewpoint that most of us have been brought up to embrace in this physical world.
There are many varied sounds in the realms of the supernatural. Sometimes the Music of the Spheres comes through. The depth and beauty of it is beyond description but if you ever hear it you’ll never forget the experience. Kind of a heavenly symphony from God.
At other times, a single sound may only be evident. The note of a flute. The gentle ringing of a bell, or sometimes, many bells. The roaring of waves breaking on a beach. A gentle breeze blowing through the trees or sometimes heavy, howling wind. Even the rumble of thunder may be heard at times. In many instances, if a person is in contemplation, the sound heard can give a clue as to what plane or realm is being visited. Your angel can supply that information if you ask. As you become more sensitive you may find that you occasionally hear some of those inner sounds mentioned above while you are in a fully awake physical state.
Keep those inner and outer ears open and sometime soon you may be blessed to hear some of those heavenly sounds. They put even the most beautifully composed music on earth in the shade.