banging steam pipes

i have a one pipe heating system in a house bult in the 1920's. I noticed at times the pipes bang...extremely loud. My plumber told me it was due to the pitch of the pipes. In a rental i have it does the same thing. Why does it do this and what is the fix???...Thanks so much

Do you have a water level gauge? you should have a glass gauge in the side of the boiler and the level of water in there should be about 3/4 the way to the top of the glass. If you don't see a water line, the water may be too high and then you would drain enough water out till you get the proper water level.
If the water level is high or a radiator vent is plugged and won't allow the water to drain back to the boiler.
You can get pounding when the steam is trying to get by high water.

banging

You get banging when the steam contacts water in the pipe and condenses violently. It can be caused by improper pitch trapping the normal condensate on its way back to the boiler, or if you have "wet steam" the excess water will do the same thing.

thanks plum.. the site glass is 1/4 full of water.. if i can hear hissing from the vents ill assume that they are not blocked...correct??...
Is this banging a problem??.. i could live with it as long as it's not hurting the system... the sound is so loud and violent...it seems like it's doing damage..