chuff

A chuff is a friendly word for the sound an engine makes when it's running smoothly. The engine "chuffs" along.

You might not hear this word too often unless you're into trains or steam engines. But if someone says something like, "The old train started to chuff as we left the station," they probably mean that the engine began its gentle, soothing hum. It's a very British term and is often used in stories or conversations about trains from a long time ago. If you've read about vintage locomotives or watched movies with train scenes, you might have heard this word before.