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Last season my heating system was noisy at the boiler; until the sysem had been cycling almost all night, when it would stop. I have been advised it was cavitation. Last season I installed a Grundfos 15-100 in place of a 15-58, thinking that more "punch" at a low flow would improve the heat at the farthest radiators. That worked as best as I can tell. But now the noise. The boiler is a Burnham 204, with a boiler by-pass, circulator pulling away on the supply side. By pass is fully open and the return flow ball valve is half closed. The provides good boiler operating temperature which is something I have wanted. Piping is 1 1/4" CI at the boiler. All large Cast iron feeds and radiators. Your thoughts and suggestions to eliminate the noise are welcome.