Sounds like a air lock noise coming from the heater core. GM has a service bulletin out for this....but be careful as hot coolant can cause serious burns. If you don't feel comfortable about doing this take it to a reliable professional.
This method should be used with due care to ensure that as much air as possible is purged from the cooling system as quickly as possible whenever the system is being refilled, or repaired for any reason.
Note that any vehicle exhibiting a persistent "gurgle" noise, heard coming from the heater core has excessive air in the system and should be purged to prevent engine damage due to extended operation with low coolant.
Air Purging Procedure:
Either with a shop evacuation-fill machine or manually, fill the system from the engine fill neck as usual until the coolant level is visible and stable.
Note: At this point the radiator and engine will be relatively full but the heater circuit will still be full of air, if it had been drained and not filled independently prior to fill at the engine fill point. If the heater circuit had been drained, you may fill it independently by disconnecting the hoses at the on-engine pipe connection and carefully filling both the pipes and hoses prior to connecting them.
Put the cap on loosely (threaded on about one turn), start the engine and without delay, raise the engine speed to about 2000 RPM and hold it there for between 45 to 60 seconds minimum, then shut down the engine.
Remove the cap and again fill at the fill neck until the coolant level is visible and stable. Install the coolant cap, threaded fully on. Also ensure that the coolant recovery bottle is filled to the indicator line
Note: At this point the heater circuit will now be primed full of a mixture of mostly coolant with a mixture of air, but the engine still has significant air trapped inside.
Now you must run the engine until it is hot enough to open the thermostat in order to loosen the entrapped air from the engine so it may be purged. In a shop environment, you can accomplish this by holding the engine speed above 3000 RPM until the temperature gauge reads between the 2nd and 3rd lines.
Once you reach this temperature, complete a series of at least three (3), four-second-duration idle -to- four-second-duration 3000 RPM cycles. This effectively "burps" any remaining trapped air in the engine to the higher location, at the fill neck. Then shut down the car and allow it to cool down enough to safely remove the cap for a refill.
Remove the cap and again fill at the fill neck until the coolant level is visible and stable. Install the coolant cap, threaded fully on. Also ensure that the coolant recovery bottle is filled to the indicator line.
It is recommended that you should thermal-cycle the system and check and refill coolant a couple more times either by steps 4 through 6 or by normal driving that allows the engine to reach the thermostat-open temperature (between the 2nd and 3rd lines on the gage), cool down until coolant can safely be added (overnight is best). You should continue this process until you are no longer able to add coolant.
Heater should be on high during the in-shop fill procedure this just keeps the car cooler for quick fill.