The term 'ominous bell sound' can be summarized by the crossword solution 'KNELL, ' which is a five-letter word. A KNELL is a solemn or mournful sound made by a bell, usually to mark a death or a funeral. It is also used to signify the end of something, such as a period of time or an era. The sound of a KNELL is often associated with sadness and foreboding, making it an appropriate term for an ominous bell sound.