Air Pockets in the Heater Core
Today's EATC systems use blend doors to blend ambient, heated, and cooled air to achieve the desired temperature. With this type of system, a gurgling noise can occur whenever the engine is running. Improper bleeding of the engine's cooling system results in air pockets getting trapped in the core.
Check for a bleeding port on the thermostat housing. On specific models, manufacturers recommend lifting the back or lowering the front end of the vehicle to remove air pockets whenever refilling after a coolant flush. If unsure, check for special procedures before performing the flush.
A blown head gasket results in combustion gases entering the engine's cooling system. It makes a noticeable gurgling sound under the dash as the coolant flows past the air pocket in the heater core. The heater core is unable to heat the cabin properly.
Air pockets in the coolant system contaminate the coolant and damage aluminum parts. As coolant breaks down, electrolysis occurs, and aluminum parts like the heater core begin to leak. The coolant leaks onto the carpet in the passenger's compartment.