How do I get rid of the banging noise from my cast iron baseboard heaters.

My home was built in 1967 I don't know if the heating system is original. Its baseboard cast-iron in the winter, when it very cold and we raise the temperature, throughout the night, it makes him loud banging noise. The plumber said (last January) there is no air in the system & the boiler is not overheating, nothing can be done. I'm hoping someone can help me, because we would like to sleep better at night.

Banging is usually caused by water expansion. Instead of calling a plumber try a HVAC professional with boiler experience. By having a on site inspection done professionally you will get the correct answer you are looking for. All we can do here for your situation is guess.

below is my guesswork

Check expansion tank and isloate and drain if necessary.
Check your system water pressure and make sure it is set for proper system height.
Check your boiler water temperature setting to be sure it is in range for your system
There are water hammer elimination devices that can help with this problem