Some of the correct measures where mentioned previously but the banging is usually associated with incorrect pitch in the feed pipe (from boiler to radiator) the banging you hear is when hot steam meets cold or condensed water in the pipe-when the two meet-BANG. you may try (assuming you pipes are asbestos free) gently re-pitching your pipes-make sure the steam goes up and returns (pipe heading back to boiler) is pitching down or back to boiler. If you don't have too many radiator vents in you system I would replace them all-It happens sometimes that people mix and match vents and that can cause an imbalance in the system. I would suggest Vent-Rite adjustable vents or Dole adjustable vents.
DO NOT be fooled by cheap hardware store or Home Depot adjustable vents for they DO NOT WORK! It pays to spend a little more on these relatively cheap items then the extra fuel and time wasted by using inferior products. With these vents you can clean them every other year by boiling them in a vinegar solution during the summer to remove sediment.