Best Of

Yes. On the inlet.

I think "Got Steam?" Is a must read for most people. I think the Lost Art is a great resource and read for those who need ALL the information. IMHO.

As an example of hope for @maroon, here's a recent example of a resident steam pro finding a steam problem that had apparently stumped other contractors, without having to open any walls or ceilings:

Sometimes the answer is in plain sight, but only for those who have the trained eyes to see it.

Where are you located? There are a number of steam pros on this site in the tri-state area who could probably diagnose it correctly. If I were you, I'd pay one of them from my own pocket to come out and find the problem. If it's boiler surging or a sagging pipe, those can be fixed. Then negotiate with your landlord about getting reimbursed, or do that first.

It may cost you, but not as much as having to move, and your sanity is worth more than money.

Sounds like hammer to me.

Hammer is steam encountering pooled water in a pipe or radiator and slinging some of that water into the next thing it encounters like a pipe elbow, or end of a radiator. So, there is a pitch or sag issue somewhere that is pooling water and causing this. It could be hidden in a floor/ceiling in a horizontal pipe run of some kind.

The insulation sounds good and all, but the sound travels through the pipes and can resonate all over, so it may just be a waste of time. It can be a challenge to locate these issues as the steam moving the water can moving it a distance and make sounds in one place, but the issue is almost certainly before that spot, how far before that spot is the question.

They aren't going to solve this by looking at other apartments' radiators. The boiler is surging or as @KC_Jones said there is a low spot in the pipe somewhere.

Respectfully to the boiler tech, they should know this, but radiators are a lot easier to show up and look at than the pipes are. I wish you good luck. There have been people on this forum in your situation whose only choice was to move.

The vital information American-Standard left off the instructions in 1952:

Back of a Sunrad Radiator: ( Face this side toward wall )

Front of Sunrad Radiator: ( Face this side away from wall )

They should have labeled the casting. So silly of them.