What does a car sound like when it’s starting?
I would go with vroom, not va-va voom, because of the dictionary definition.
va-va-voom in British English (ˌvæˌvæˈvuːm ) NOUN informal the quality of being interesting, exciting, or sexually appealing
https://www.collinsdictionary.com/dictionary/english/va-va-voom
Collins English Dictionary. Copyright © HarperCollins Publishers
Another reason why: we say an engine “purrs”. A purr, by definition, is a vibrating noise or hum. “Vroom” definitely is a vibration, but not va-va voom.
- VERB When the engine of a machine such as a car purrs, it is working and making a quiet, continuous, vibrating sound. Both boats purred out of the cave mouth and into open water. [VERB preposition] The sleek car purred down the country road. [VERB preposition] [Also VERB] Purr is also a noun. Carmela heard the purr of a motor-cycle coming up the drive. [+ of] https://www.collinsdictionary.com/dictionary/english/purr Hope that helps.