Bubbling as in foaming? If that's the case it the coolant would be contaminated. Do an acidity test on the coolant.
BHG can vent gas from the cylinders into the coolant or oil passages without allowing either to leak into the cylinder. Very small leaks will do this. If there's a pinhole leak in the head gasket a compression test or leak down test usually won't reveal it.
To burp the coolant system you need to raise the front end of the car as high as possible. Remove the rad cap. Then start the car and turn the heater on. Air often gets trapped in the heater core. Run the car for just a couple minutes allowing any air trapped in the system to exit. Turn the car off. Allow the engine to cool thoroughly and add coolant if needed. Then replace the rad cap, top off the overflow tank if needed and lower the car.
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