Eliminate the knocking sound? But that’s the beauty of a radiator! Knocks and hisses were music to my ears on a cold might. I don’t know how many times over the years I drifted off to sleep with the radiator knocking and hissing in the background. Now I live in a place where it’s warm all the time and radiators are not needed. I love it here, but I sometimes miss the old place. It’s one of the things I miss the most!