I couldn't guess at the boiler water level and I don't even know what kind of system you have such as a one pipe or a two pipe system. If you have any mechanical abilities at all, the articles, etc. at that website will tell you all you want to know.
That said, anyone with a boiler system should have a regular service contract with a plumber/hvac person who specializes in them. They need annual cleaning and maintenance and they are such finely balanced systems that you need someone who understands them to keep them tuned and working right. Otherwise you end up with the bang and clanging. Good luck.