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The guys talking about the ZV's are probably on the right track. A boiler jacket "thud" will not resonate through the piping (duh). Are there an other variables on the job regarding water flow, i.e. blending/temp. control valves? You may also want to check the hanging/support of the piping/radiation itself; bell hangers especially (if tight)may cause a noise as piping expands/contracts. This movement cannot be halted and must be accounted for (cast iron baseboard gives such a noise). Hope this helps.