The sound you are hearing is the result of air trapped in the cooling system. Sometimes will be worse after a cooling system flush and refill if all the air has not been removed. As stated above, a very common problem. After much customer complaining, years ago mercedes developed an "anti-gurgle" kit. When I priced the kit a couple of years ago at a dealership it was something around $800!!! Mine will do it occasionally, sometimes months between occurrences. It has been suggested that you may be able to purge the air by parking the car on a slight incline, then venting any trapped air by loosening the bolt on top of the thermostat housing. Before doing this, be certain to have operated the car with the climate control temp. wheel set to the warmest position. Be cautious, if the car is up to normal operating temp., the coolant will be hot and pressurized! Often the sound is heard only while the coolant is below full operating temp. If mine does it, I turn the climate control knob all the way up to the warmest position for several seconds, then back to my comfort zone setting.
OliverB