Make a rain object first.
-In the creation code for the rain object, you can set a single v-speed or make it choose from a random v-speed.
Then make another to spawn a rain object across the x-axis of the grid in each room.
-In the creation code for the spawn, set an alarm for how frequently you want a rain object to spawn on the x-axis.
At least, that's how I do it. Hope this helps