When there is bubbles in the cooling system that is not going away after many days of driving, one may suspect a blown head gasket. However this is accompanied with other symptoms such as white smoke and/or water in oil/cooling system getting pressurized,/water reducing in the system etc.
In your case it is not for sure that the head gasket is an issue or that Guard coolants are frothy. Just as an academic discussion of head gaskets, have a look at this sketch I made:
This is a representation of an engine block and the head gasket.
There are four straight lines, yellow, red, green and blue. Each line represents a mode of gasket failure. The yellow line is a failure between two adjacent cylinders. Usually indicated by poor engine performance, poor compression in these two cylinders, failure of leak down test, lumpy idle.
The next failure is the green line. This is a failure between the water and oil passage. Indicated by oil in the cooling system and/or water in the oil.
Next is the blue line. Failure of the gasket between the cylinder and oil passage. This is indicated by high crank case pressure. Excessive oil vapor in the PCV system especially where the system connects to the intake manifold.
Lastly we have the red line which connects the cooling system to the cylinder. Here the combustion gasses enter the cooling system as the pressure of the combustion gasses is higher than the water in the cooling system seen in the form of bubbles in the system at the radiator neck. When the engine is switched off, the water can go and enter the cylinder and burns off during the first start of the day with a tell tale puff of white smoke from the tail pipe. Also engine running temp will be observed to be high due to gas bubbles in the system causing air lock.
So from this study I leave it to you to investigate further.
