Why do I hear strange noises when there's nothing around me, not even a dungeon in minecraft?

When I was building an underground home in Minecraft, mind you I had mobs spawning off so when I tried to spawn a zombie it wouldn't appear, I heard footsteps when I was looking in my inventory. It scared the heck out of me and I'm afraid to reenter the world. What should I do and what is happening in there for there to be no noise but those footsteps? I looked for hundreds of blocks in caves and just digging around, but I couldn't find the source.