Halo 3 Easter Egg - Secret Vehicles

on the level halo play it with 4 players and legendary on. go to the part where sargent johnsons warthog is left at the snow. go past the hidden mongoose and when you see the sky through the cliff you will see two rocks. look through them and then you will see two brute choppers. now heres the other vehicle. go to rally point bravo and go to the part where you see the first two sentinals. then turn left go down the stairs and turn around there will be four ghosts.