Advancing to morning (when using a bed) when it is dark enough for mobs to spawn will always result in various amounts of zombie "rain" in the morning. The zombies will spawn somewhere in the sky, fall to the ground, receiving no fall damage, and begin to burn. No other mobs have been seen "raining". Discovered in survival (not a fun surprise), and tested in creative. 'Hard' difficulty and 'Far' view range.