The Sound of Silence

There is a sound even in the dead silence.
In the very late night or early morning, after all the daily sounds such as the sounds from street, refrigerator, TV or radio are taken away, only there left is the very high pitch, metallic, severalfold sound -- the sound of the air. I call the sound as the sound of the air, because I can't find any source and it is heard in everywhere. ・・・May be this sound is the crushing sound of particles in the air. I only guess. But I like this sound.