The radiators are banging because water from the previous cycle did not drain entirely out of the radiator to return to the boiler. When the steam rises it hits that cold water and bangs. To fix it, raise the silver valve end of the radiator an inch or so. Place wood blocks under the radiator legs. This will tip the radiator slightly towards the pipe that goes through the floor, so the condensate will drain back properly. This will only work in a one pipe system though but should solve the problem.