"What's under hood" is a question that we often use while talking about cars and vehicles. It refers to the mechanical parts and systems located beneath the bonnet or hood of a car, such as the engine, transmission, and cooling system. However, there are many other synonyms we can use to ask this question, such as "What powers the vehicle?", "What's behind the grille?", "What's under the hood or bonnet?", "What's beneath the engine?", "What's the power source?", and "What's under the chassis?". These synonyms help us to find out about the technical details of the vehicle and understand its performance and capabilities.