Generally, with a hot water heating system, you get banging as the hot water starts coming thru and causes the cold pipes to expand and contract. As they expand and contract, they also might rub against whatever brackets are securing them in place.

Pribuss said that is likely to be specific to the units and those particular radiators. Meaning that there may be a problem with the valves that let the hot water into the radiators. He said sometimes valves are put in backwards to the water flow, and that can cause the banging. 

It may or may not be the issue he described, but any owner who has this problem would have to contact the plumber and they would have to check the individual radiators and the valves. If there are several units with this problem, they probably should be scheduled as a group. This could be done by Hetherington or Pribuss, whoever the owners prefer.