Water Heater?

My water heater, for the first time since I have moved here (a month) was making a sound as if it was either filling back up or draining water. I'm not sure how long it lasted, but it was the first time it did it. I don't know if I should be concerned or not. Is it normal for water heaters to occasionally make noise? It isn't doing it right now. Thanks!

If it was electric it sometimes will make a similar sound to what you described. Gas on the other hand will some times sound like a bunch of marbles are rolling around inside.