Goofy yell tribute
From Halopedia, the Halo wiki
The Goofy yell tribute is a yell resembling the Goofy yell reportedly heard in the Halo 2 Campaign level High Charity, as well as in Valhalla and High Ground in the Halo 3 Beta. The sound effect is actually a blooper from a Halo: Combat Evolved voicing session where Steve Downes was voicing death screams for the Master Chief.
On High Charity, the player would have to climb onto a gravity lift near the end of the level. The player would activate a switch, wait for the lift to rise a bit, then slip underneath it. The game would detect the fact that the player was not in reach of the lift, and would lower the lift onto the player's head. The player would then occasionally hear the Goofy yell. The laugh can only be heard in single-player or Xbox LIVE; Co-op has not yet been tested.
On Cairo Station while the player is using the Mac-cannon glitch, it can occasionally be heard while you are falling through the floor to the air lock at the end of the level.
In the Beta, a player played on Valhalla with 300% speed and 50% gravity. The laugh was heard when he jumped a series of rocks leading out of the map. After that, the player leaped into the water and died, hearing a scream sounding like Goofy. Also in the Beta on High Ground, the laugh was heard at the end of the match.
The ‘Goofy Yell’ has also been reported being heard on Coagulation via local 2-player multiplayer after the second player was ran over with a Wraith.