with out hearing it for my self it hard to tell , if you can pin point were it coming from would be good , try a stethoscope . is the water pump working right , radiators fittings all tight , r cap sealing good, not overheating , check head for straightness and cracks , were was it leaking before ? what sought of scaring did the piston have and why did it have the scaring ? did it over heat badly before you change the piston ? is the coolant level dropping on a long ride , it most likely going in the combustion chamber which will make the ring lands break , if that the case , take it apart and check everything with a fine tooth comb.