It is possible to replace the jets with flying classic cars. Something can be done in novice mode at level 6 that will result in a constant tone until you get to level 7. Level 7 will then start with classic cars flying at you instead of jets. The level passwords with the cars are identical to those with jets. You must duplicate some sort of game condition (complete all the first six levels with no damage? Complete level 6 with no damage and using only one plane?) to unlock the cars.