It's most likely air trapped within the engine block.
With the motor completely cold, remove the radiator cap.
Start the engine and let it run with the cap off until the radiator fans come on.
During this time, with the coolant system unpressurized, any air bubbles will circulate up to the filler neck on the radiator and pop. If there's a lot of air in there, you may need additional coolant.
And as always, you don't want coolant to dry on any painted surface.
So don't go walking away.
