Steam Radiator Making Hammering Noise
It is called cavitation and will eventually destroy your system.
Cavitation is cause when moist air is superheated and expands, then comes into contact with something cooler and collapses. When it collapses it causes the hammering noise you hear.
Cavitation is one reason that older submarines could be heard. That hammering could be heard for miles.
This process does cause metal loss and is the major reason that propellers on boat motors need to be replaced. The other reason is drunk boaters who can't see the shallow waters full of rocks.
So how do you fix it.
First off bleed all the valves WHEN THE WATER IS COOL. I don't want to be responsible for thermal burns here. Every radiator has to be bled.
Then make sure you have an air scoop in the system and that it is working. This will get any excess air out that you can't get by bleeding the lines.
There will always be noise in these types of systems but with some work you can minimize it.