Oil-Fired Steam Boiler, What are causes for Knocking and Scratching in Pipes and Radiators?

Thank you for reviewing my question. I have an oil-fired steam boiler, a Peerless WBV-03-110. It is in the basement and it heats a 2 story residence, about 1000 sq. ft. total. There are three iron radiators on each floor. Occasionally, I have had a problem that occurs about halfway through the heat up cycle where I can hear some water rustling and scratching in the pipes and radiators. Eventually there is some light knocking which gets louder and heavier. The retard on the boiler which usually reads between 0 and 2 PSI holds steady between 5 and 8 PSI, even an hour after the cycle is complete. Do you have any ideas what causes this? I suspect a problematic vent on one of the radiators, but could there be other things I should look for? Thank you for any istance.