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I have a 3 year old Navien ncb240 that recently puts out a kettling(banging) sound when domestic hot water is being used,I had a plumber come to address the issue and he said the boiler was installed incorrectly. he said I lacked a looped manifold and magnetic filter. He also said my 2 gal expansion tank was too small.I flushed out the system(heat exchaner) but the noise is still present. The plumber wants thousands of dollars to do the proper work and said the kettling may or may not stop . What next?