Steam Surging

[Patrick Carle]

I have an old 1 inch fin on the first floor directly above the gas boiler. A Number D radiator valve is screwed into a cap at the end of the pipe. Here's the problem - I always have a moderate surging sound of steam coming out of the valve when the system starts (not a good sound to begin with). But occasionally, I am now getting a large surge that is really rattling the pipes. The sound is different than water hammering. Any help appreciated. Thanks...