Why does everything around you seem to get louder when you doze off?

Brain winding down: It's probably from your brain's audio filter shutting off. As we drift off to sleep our brains' environmental filters slow down. Audio signals we normally ignore and don't pay attention to all of a sudden become louder. It's the opposite of when the air conditioner or heater shuts off and you realize it was running because you no longer hear the noise.