Harold Kestenholz
The cause of the banging in your water system pipes is a best guess from any distance. This problem needs to be examined by a qualified, experienced heating plumber.
A pump, pipe, hanger combination can set pipe in motion to cause knocking.
Air in the pipe can move columns of water through pipe to bang against fittings. Usually you will hear water rushing noises along with the banging.
The boiler itself can have pockets of steam develop that will send banging through the pipe system.
All of these need to be checked by a competent service person.
