i did not pay attention to the water levels of my maintenance-free Benz-original (white) battery. Maybe the water level was checked at the Benz service center when i last brought it in for major maintenance service.
it did not make the series of "gurgling" noises like yours. But it did make ONE noise. One day i was starting up the car, and the battery exploded (!). The top of one of the cells was gone and there was some acid spilled in the area. My guess is that the water level was too low and the hydrogen present there was ignited in some way.
make sure the water levels are normal, even if it is maintenance-free.