The phrase "what's under the hood" is often used to inquire about the engine or mechanical workings of a vehicle. If you're looking for synonyms for this phrase, some options include "what's powering the machine," "what's driving it," "what's propelling it forward," or "what's making it go." Other variations on this theme might include asking about what's "under the bonnet" (a British English equivalent), "what's under the chassis," "what's under the hood ornament," or even "what's under the engine cover." Regardless of how you phrase it, the question remains the same: what's making this vehicle run?