Baseboard heat register makes loud banging sounds
It sounds like there's water in your system. When you have a hot-water based radiator system, sometimes air will get in there and start making banging noises.

If it is a water boiler, I also highly suggest getting a new tech.

When I bleed (let the air out) of my radiators, I turn the system on and let it run at a hot temp, then turn it down and let it cool off. Starting at the radiator closest to the boiler I turn the bleed valve and let all the air out until water starts to come out. It helps to have a bowl or something to catch water when it comes out.

Repeat this process until you make it to the top floor. You may have to repeat the process again in a day or so, but this should take care of the issue.

If you need a more detailed guide read here