I also just returned from a cruise and was in the front of the ship and heard similiar noises. I got up to see what it was and figured out I was below the aqua pools (spa area) and it must have been the water in the pools splashing out .. It was different than other times I've cruised but I was never directly under a pool before, It then sounded like later in the night they were draining the pool because I could hear a pump with water running. this was on the Infinity and it was only one night on an inside forward cabin.