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Pipes in a hot water system only bang when the water is too hot (near or above 212) and there is air in the system. When the too hot water meets an air pocket it flashes to steam and BANG. I'd suggest you bleed the radiators again and either turn the water temp down or replace the aquastat that is controlling the water temp. An old hot water system with radiators typically functions with water in the 160-170 degree range. Well below the boiling point.
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