No. Sometimes the boiling of the coolant from exhaust gases escaping the combustion chamber IS seen in the overflow tank. Either way, if exhaust is bubbling the coolant, the most likely cause is a blown out head gasket.
And, with the radiator cap off, as I suggested earlier the coolant should not be building pressure, aside from coming up to temperature when the t-stat opens; therefore if the radiator overflows there is pressure present from the combustion chamber(s). A normal operating engine will not overflow the radiator when the t-stat and water pump are functioning correctly. Instead a flow of coolant past the filler neck will be present- just like a river running through it. Pressure is created by the presence of the radiator cap sealing the system. When the fluid expands under this normal set of circumstances, then the expansion of the coolant goes to the expansion tank.
