Hissing sound from certain radiators after heating

jk914

Hello and thank you in advance for your expertise.

I have a 100-year-old house with a Burnham steam boiler that is less than ten years old. I have no problems with hissing, gurgling or getting temperature to each radiator on my two stories while the system is heating.

After heating, which I presume is when steam condenses again, certain radiators make a very loud hissing sound at the air vent. With my limited knowledge (so far) of steam heat, I understand that condensation happens very rapidly and I think the system is searching desperately for air.

Last season, I replaced all radiator air vents with Varivalves but did not touch the main vents-- my hissing issues did not exist last season. This is a single pipe system but I have two branches at the boiler: one for the front side of the house, one for the rear. The two branches rejoin in the basement and return to the boiler with pitch. It looks like I have two main vents, (I think), located at the end of both runs before they rejoin.

Let me know if photos would help and what, specifically you'd like to see.

Thank you