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older American Standard cast iron boilers--appearantly a close spacing in sections that scaled up and actually causes small bits of water to explode into steam, I have changed out several of them over the years, and have seen many of the same era and model that do not have the problem, American Standard quit making booilers shortly after this problem started showing up (no absolute connection) The boilers were jacketed in light brown w/ a black Diverter. The problem usually starts out as a rather quiet popping sound and eventually gets very loud and annoying albeit not dangerous. Replacement seems top be the only remedy. I once came upon a very noisy ( same kind of sound) in an old Weil McLain boiler and using a pump/bucket system recirculated a strong solution of muratic acid through the system for about four hours. the noise went away and has never returned (that was 8 years ago). The house belonged to a freind and he says all is still fine. I wouldn't necessarily recommend this as it was very messy and required lots of time and effort to bring the system back to proper ph etc.