bike starts up easily, first kick, coolant level is topped up, ride bike for ten mins or so, then as soon as I turn off the engine the radiators go ''HISS BANG, BANG! HISS! BANG!'' for maybe 20 seconds. Then coolant comes out the overflow pipe, in short bursts for maybe ten seconds, a considerable amount too. I let bike cool down, retopped up the coolant which was now below the top of the fins, rode again, same thing happened. I can't see any leaks on the cylinder, the spark plug and head feel dry, the base gasket appears fine. No visible leaks from either radiator or the hoses and all of them are warm/ hot when bike has been ridden. I rode for 70 miles a few days back, and the same hiss bang thing happened when I filled up with fuel, and that time it was almost empty of coolant by the time I got it home to fill the rads back up. What is going on??