Bubbling (not boiling) coolant

Generally bubbling coolant where the temperature is not high enough for boiling indicates a leaking head gasket, a cracked block, a warped cylinder head, or something similar that allows exhaust gas to enter the cooling system.

After opening the system to change the thermostat, you should have purged air from the system using the procedure described in the service manual for your vehicle.

If there is still air in the system, it should work its way out as you're seeing it bubble out.

But if it continues then there is a problem as noted above.

You probably didn't bleed the system properly. First of all, let the engine cool down completely. Then, open all bleeding screws and add coolant to the system until clear coolant without bubble comes out of all bleeding holes. Tighten all the screws in the order of lowest to the highest because air exits through the highest point in the cooling system.

After doing this, remove radiator cap (or reservoir tank cap) and put a relatively big funnel in the radiator filler neck. Pour coolant into the funnel so that the coolant in the funnel becomes the highest point in your cooling system.

The final step is to start the engine, keep a steady 1500 RPM and turn your heater on (because coolant should circulate through the heater core too). Let the engine run until it gets up to its normal operating temperature. You will see lots of bubbles coming out of the funnel during the process. Let cooling fans turn on and off at least 2 or 3 times. When you didn't see any bubbles, you can turn off the engine and adjust your coolant level on the reservoir when the engine is cold. If you still see bubbling after bleeding, you may have a leakage somewhere or a problem with your cylinder head or block.