You're in a cavern. It's dark. But your ears work fine, so you know you're safe. All mobs make noise, right? Zombies moan, Spiders squeak, Skeletons' Bones tinkle, Slimes splat, Endermen... make... Ender... Noises? Except... for the dreaded CREEPER, who makes no noise until it's TOO LATE! But... Do creepers make noise when they walk?
No, seriously. Do they? I'm in a cavern and I'm hearing something else taking steps, however I'm not hearing any other noises associated with mobs. Is this a Creeper creeping around somewhere?