• Air pockets in the heater core can interfere with proper coolant circulation. Air pockets form when the coolant level is low or when the cooling system is not properly filled after draining it. 
• A gurgling noise in the heater core may be caused by a low coolant level in the cooling system. A low coolant level will allow too much air in the cooling system. The excessive air will mix with the coolant and create the gurgling noise. 
• A restricted heater core will also cause a gurgling noise from the coolant passing through the restricted area. 
• Coolant leaks. Internal engine coolant leakage is probable the worst-case scenario. 
• The engine thermostat is staying open and the vehicle doesn’t reach operating temperature. 
• Heater control valve malfunction. 
• Blend door actuator motor malfunction. 

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