The headgasket when breached can allow exhaust fumes to be compressed into the cooling system.
The headgasket is between the cylinder head and the engine block (on top of the cylinders). There are coolant passages nearby that can be exposed to the compression of the cylinders if the gaskets fails there.
The coolant continues to circulate carrying the air bubbles until the surface.
This may not be what is happening, but MAY be.
Get it looked at.
Tuesday, December 26th, 2006 AT 7:48 PM