Baxi Bermuda back boiler won't turn off?

I have a Back Boiler about 25 years old. Recently when the boiler has been on for two or more hours I start to hear alot of loud sounds in the loft like water, thunder and banging, but mostly water. I turned down the thermostat but the boiler wouldn't turn off. I turned the timer to off, still the boiler raged. I even turned down the tempreture dial on the boiler, that did not work either. In the end I pressed the red on on the boiler, which did work. I've drained the system and changed the thermostat, but the problem has not gone away. Can any one help?