There could be many reasons why your boiler is making noise and if you have no experience in fixing boilers you will want to call a heating professional to come to your home or business to fix the boiler for you.
The Circulator Pump on the boiler is what will transfer and pump the water through your heat loops and baseboard elements. When the circulator pump goes bad it can make some noises. Depending on what type and brand of circulator pump you have will determine what has failed and now making noise. If you have an older B & G circulator pump the motor / pump coupler could have gone bad and you are now hearing a rattling sound. Sometimes in other brands of pumps the motor bearings will start to seize and make a loud noise. The boiler repair man can replace the circulator pump for you in anywhere from 60 minutes to a few hours depending on how your boiler is piped. The cost of the new circulator pump installed can run between $175 -$800 depending on the type and model of pump you replace.
Water Hammer could be another reason your boiler is making a banging noise. Water hammer happens when hold and cold water meet each other inside of the boiler or the heat loops. You should hire a licensed boiler repair man to help you fix this type of problem. This type of banging noise can be fixed by the heating professional as well. If you are hearing a banging inside of the boiler pipes (heat loops, heat lines) you could have low water inside of the boiler and you could be hearing the water roll around inside of the heat lines / loops. You can have the boiler repair man bleed all the air out of your heating lines to prevent this from happening in the future. After a long off season it is normal for the boiler to loose some water in the heating system from evaporation.