There can be numerous causes of coolant bubbling in an expansion tank of a car. Most of the time, it happens when a car is experiencing overheating.
Sometimes, coolant in expansion tank can bubble as if it is boiling even when the engine is yet to attain operating temperature. It also might be accompanied by a popping sound. Diagnosing this kind of situation can be very tricky because when the engine is left to run, it will eventually overheat making whoever is diagnosing the issue to look at problem areas like thermostat, radiator and so on. Unfortunately, those are not part of the causes of bubbling coolant in expansion tank when the engine has not attained operating temperature.
Note that little bubbles in the coolant reservoir or expansion tank can be normal and might not signify a problem.
These are two major reasons why coolant in expansion tank bubbles when the engine is still running cold
1. Head gasket leak:
A head gasket leak cause combustion gases to escape into the coolant and a leaky head gasket can cause this to happen even when the engine has not reached its operating temperature or when it has not overheated yet. The escape of those gases into the coolant is what causes bubbling that is seen in the coolant reservoir or expansion tank. To remedy this situation, the head gasket will need to be replaced.