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The valves on that radiator suggest to me that this may have been piped as a two pipe air vent radiaator. One pipe generally led to a steam main. The other to another pipe which also had steam in it, but was intended to carry condensate back to the boiller as well. A return, if yu will, but not a dry return as would be found in later systems.

It works. Actually, in many ways like a one pipe system. However, pipe pitch — not so much radiator pitch — is essential to make sure that condensate can, indeed drain.

So that is where I would start looking. How does condensate drain? Are the pipes all pitched to allow it to drain freely?

Also make sure both valves are wide open…

I would also include the EDR calculation with the boiler manufacturer's recommendation for sizing according to the EDR. Just in case you actually need a 4 section boiler to get it to operate properly without being undersized (and the inspector will only approve a 3 section boiler that more closely matches the building load.) Unlikely but power can be intoxicating.  

OR

You can always make the manual J come out to the BTU that you need it to, by increasing the infiltration number to 2 or 3 air changes per hour.  You won't believe how that increases the load calculation numbers. 

Don't sweat it. Make good general estimates of al that sort of thing — no need to get carried away. Remember too that this Manual J is for the building code folks — since this is steam, you are going to size the boiler to the radiation anyway, not the building heat loss.

I would say that is not normal, my Big Mouth is petty quiet last I checked it. Maybe the 'O' ring needs replacing.

I'm gonna guess it's not the bigmouth, but the main dragging itself across a support or hanger somewhere as the main expands, or contracts, find the binding spot and slip in a piece of milk carton plastic to allow for a slipp