I think it's supposed to have more pees and arrrs in it. You kinda guessed it, it's a motorcycle thang, but it's more than just making a motorcycle noise, IMO. It's when you're riding along and goose the throttle. Sometimes just for the sound it makes.

To best experience that type of brraaappp, stand in a tunnel while a biker gang goes through it. Many of them are sure to brraappp for you.