Radiator noise is a common problem in many homes and businesses. It is caused by air bubbles passing through the radiator, which creates a hissing sound. The crossword solutions for the term 'radiator noise' are HISS (4 letters) and SSSSS (5 letters). HISS is an onomatopoeia that describes the sound of air bubbles passing through the radiator, while SSSSS is an abbreviation for the sound of the same phenomenon.