Help with loud banging and knocking

Banging is usually an issue with the piping leading up to the radiation, not with the radiation itself. Unless it's leaking changing that was a waste of time and money. Someone working on your system doesn't know what they are doing.

Banging (water hammer) is a sag or incorrect slop holding water from draining properly. The steam picks up the water droplets and "bangs" them into the next hard thing they encounter, typically and elbow in the piping.

Piping needs to be looked at for proper slope and not sagging. Fix that and most water hammer issues go away. There could also me issues with carryover into the system causing excess water to be where it shouldn't be. That is typically an issue with boiler piping, or oils in the boiler water from a new install or repair of some kind. Or it's a combination of both.

There is also expansion noise that can seem like water hammer, but it's different. That is when a pipe is rubbing on wood somewhere. The wood tries to restrict expansion of the pipe until the friction is overcome and then the pipe makes a sudden and dramatic move and sort of pops.