One of the most common causes is a blown head gasket, in which the air pressure inside the cylinder heads is transferred to the cooling system. This escaped air causes bubbling in the coolant/antifreeze reservoir, which can often be mistaken for boiling.
Most likely due to a warped head or failed head gasket leaking compressed air/fuel mixture into the cooling system. ... If you are getting combustion pressure into the cooling system, coolant will come out.
A faulty thermostat that causes sporadic opening and closing can cause a churning and bubbling effect seen in the radiator or expansion reservoir. The rapid closing and opening of the thermostat valve can also cause a pounding noise inside the radiator, due to the slamming pulses of coolant.
Leave the radiator cap off, turn on your engine, and let it run until the radiator bleeds out air. It may take between 15 and 20 minutes for the engine to heat to the proper temperature and begin cycling coolant through.
Wear a pair of plastic gloves or use a rag to feel the temperature of the top radiator hose. If the top radiator hose is cool, it means the thermostat is closed and not allowing circulation. Feel the bottom radiator hose; it should be cooler than the upper hose, since it passes cooled coolant back into the engine.