"Brum Brum"\"Bun Bun"

"Bun Bun" or "Brum Brum" is the sound of a motorbike revving that petrolhead Atsushi Kotoge from FULL THROTTLE uses. In Japanese onomatopoeic sound the word is pronounced and translated as "Bun Bun", but in English "Bun Bun" is slang for the word "Bunny", so it becomes "Brum Brum". 

The differences in spelling come from Western Roman and the Japanese Katakana alphabet, the Japanese have a "ru" and "n" sounds, but they do not have an "br", so the word changes to "bun bun" (on a personal note my favorite is "Do-den-ka-sha", which is the sound a train makes when travelling along tracks).