Purrs, as an engine

The answer to the crossword question “Purrs, as an engine” is HUMS. This is because the word “purrs” is often used to describe the sound that a contented cat makes, and similarly, the word “hums” is used to describe the low and continuous sound that an engine makes when it is running smoothly. Therefore, “Hums” is the most suitable answer to the crossword question, as it best describes the sound that an engine makes when it is running smoothly, much like how a cat purrs when it is content.