Bumper Phantoms can be found in the Halo 2 level The Arbiter. When you start the level, go directly behind you to where the Phantoms are leaving. Go as far as you can back before going near a cliff. Zoom in on them with your binoculars and you should continue to see them. After about 5 seconds of leaving your view, one of the Phantoms will slam into the other, causing it to spin out of control. The other phantom continues to chase it down as both crash into the tower below you. After which, the ships will fall down to the Malestrom and spin continuously.