Engine coolant is a fluid that is used to tranfer heat from the engine to the radiator where it is dissipated into the environment through the fins of the radiator.
Expansion tank is a tank/container that is used to give room for expansion of coolant when the coolant has attained a temperature that causes it to expand. There is also overflow tank which is different from expansion tank. An overflow tank is used to catch the coolant that overflows when the coolant expands. The overflow tank is not pressurized like the expansion tank.
Bubbling is formation of spherically contained volume of air in a fluid.
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.
2. Waterpump turning in the wrong direction:
A water pump that rotates in the wrong direction can be caused by wrong routing of accessory belt which is commonly called alternator belt.
This error is possible if the wrong size of belt is fitted by bypassing the necessary rollers or pulleys and the belt routed to areas it is not suppose to pass.
A waterpump that is turning in the wrong direction will pump coolant towards the expansion tank instead of pumping coolant from the expansion tank. This will manifest as bubbles in the expansion tank. It can also be accompanied with loud popping sound.
To fix this situation, the correct size of alternator belt will need to be gotten and the belt routed correctly so the the waterpump can pump coolant in the right direction.
You can read more on Incorrect serpentine/drive belt routing causing high temperature.