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Hi, I'm Tim and I'm an HVAC expert here to assist you with your questions. Please provide all information I request as it makes it more difficult to troubleshoot something when I'm not there onsite. Essentially, you become my eyes and ears. Thanks for the opportunity to help. At the end of our dialog, you will have a chance to rate my service, but please wait until we are finished and you're satisfied you have all the information you need. Please be patient as many times we work with several customers at once.
Knocking sounds on steam systems are usually caused by the steam and condensate mixing. Your condensate return system may not be working properly.
Well, steam systems are set up a number of different ways. But usually there are steam traps or some way for the condensate (hot water formed when steam condenses), to get back to the boiler. This system may not be working properly.
If you have cast iron radiators, there should be a air vent on the radiators. These may not be working properly. This web page explains how these systems work and their components.
On this page, near the bottom it shows a two pipe system, which most of the newer systems are.
It shows the way your condensate returns to boiler. This is one possibility why the system may be noisy. The other one is the air scoops.
Does that answer all your questions? This may be something you may have to get a pro involved because these systems take experience to troubleshoot.
Did you have additional questions on this?