What creates the loud rumble of 1960s muscle cars?

@Bing, I saw one of these systems on MTV’s show Pimp My Ride years ago. I think it was part of the sound system and worked with speakers mounted under the car or in the engine compartment.

I did a search and this is all I could find about this kind of system: jalopnik.com/137352/electronic-impressionist-mimics-car-engine-sounds

Here is a link to the actual product: vroombox.com/

Before you all start telling me that this is a stupid product, I agree. I’d never install one of these systems on my car.

No good ideas left anymore. I think the Vespa one would be a good choice for Prius.

@Whitey