More than likely, a pinhole in one of the heater hoses or water pump bypass hose. Also the thermostat water neck is kinda prone to rustin through from the inside. Best thing would be to try and rent a pressure tester from pep boys, checkers etc.. and pressurize the system and then look for where the waterfall is....lol
this x2 (also a mechanic)
but since everyone else is throwin out random ideas... lift the radiator cap, drive a whole new car under it, replace cap. PROFIT!!! //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif