If you start the engine and quickly look at the surge tank you will notice a significant drop in coolant level. I believe this is what is causing the gurgling sound, if air is getting sucked into the heater core. Mine just started doing it about 3 weeks ago. My coolant level was about a half inch low.The first time I mentioned it to my dealer they said they heard it but there is no solution from GM (Feb 2017). I switched dealers for service and mentioned it again at my final warranty work and they said they could not duplicate it but if it happens again or becomes super noticeable, they will repair it because I brought it up during warranty time frame. And by repair it I mean replace it with the new "surge bottle" that GM released about a year ago or so.
I then just researched the forum and found that topping it off with some dex cool 5050 slightly above the cut off line eliminates the water sound or minimizes it by 95%.
Dexcool doesn't play nicely with air in the system. Things have improved since the introduction of Dexcool, but it does make you wonder about failed heater cores.