The radiators in my 70 year old house knock (same sound as air in the pipes) when I walk by. This happens whether the radiators are hot or not anytime of year. The noise goes away for a while then knocks again when I walk by hours later. I did move the radiator pipes around in the basement a few years ago (b/c I moved the boiler) but I do not remember if it made that noise before then. Any thoughts?