Airwolf (NES, Kyugo Boueki)

Airwolf
Developer: Kyugo Boueki Publisher: Kyugo Boueki Platform: NES Released in JP: December 24, 1988 This game has a hidden sound test. This game has a hidden level select.

Airwolf is a port of the classic arcade shoot 'em up with a few improvements, including an actual story. So of course, Acclaim ignored it and instead released a boring flight simulator.

Sound Test

On the title screen, press Up, Down, Left, Right, A, B, A, B on Controller 2, then press A + B + Start on Controller 1.

Stage Select

On the title screen, press Up, Down, Left, Right, A, B, A, B on Controller 2, then press A + Start on Controller 1. The intro will play normally, but after the intro, the game will jump to a stage select screen.

(Source: CaH4e3)