I can hear this noise in my cabin also. Cap is new, radiator is new. Car hits 90 c because I need a new fan clutch but normally stays at 82. The noise is not coming from the exp tank though, it's coming from behind my firewall. Is this also indicative of a coolant prob - poss related to the thermostat? I do not know if it was replaced. Coolant hoses could also be replaced in next 30k also.
Also, is there a way to determine the right coolant mixture? Can this be determined by the reading on one of those coolant temp droppers with the little colored balls inside?? I mean, if the reading is consistent with specs I would assume the right mixture and if it were below spec - add coolant instead of water??