I have recently bought a Escort RST and when returned from taking it for a test drive i heard the water bubbling and boiling in the header tank. When i asked the previous owner he told me that is has only done this since the new stainless steel tank has been put on there. Is this true or is there a problem? I have checked for the common head gasket failure signs but no oil in water or water in oil.

any helped gratefully apreciated