radiator starts banging seconds after turning on the valve
That's thermal expansion of the pipes as the hot water starts to move through them.  This is reinforced by the fact that it starts to slow down once they approach a temperature equilibrium.  It's the same effect as when you turn off your car after driving somewhere and you can hear the "pinging" from the engine bay and exhaust system; that's the same thing only in reverse (as the metal starts to cool, it contracts, causing that noise).