This is an Easter Egg that appeared in the Halo 3 Beta.
How to find it
To (if you're lucky) produce the egg, make a party of four in matchmaking and enter the 2v2 playlist, or produce a custom game. Make sure that none of the players have the intent of actually killing eachother, so you won't have the annoyance of enemies shooting you while reproducing the egg. Keep vetoing maps until you find a game in High Ground. In the Base, there is a room with two large computer systems that can be destroyed. Next to the larger one (near the door to the bridge to the pipe room) there is a small green box. Upon closer examination with a sniper scope, you can see that it is a radio. Now, here is where the luck part of the egg comes in. Take out an Assault Rifle and shoot the small group of 4 silver circles on the right side of the green colored screen. If shot just right, the radio will stop hissing static and play what sounded like the song from Halo: Combat Evolved, Under The Cover Of Night. Otherwise, it will start playing something much more likely to happen, a message in morse code. I do not know what the message says, so it's up to you guys to find it! Happy Finding!
It is unknown as to whether it will find its way to the final game.
Halo 3
It is now known that this has been changed by Bungie. To hear the music, all you need to do is wait for while in the computer room. The radio below the larger computer will eventually stop making static and morse code and will play random Halo songs (Halo 1, 2, or 3). You can tell it is about to play a song when it sounds like an FM radio just barely out of tune from a local station.
It should be noted that the morse code itself is also an Easter Egg, and, when translated, says, "Frog Blast the Vent Core", a random line from an earlier series made by Bungie, Marathon.
Trivia
- On the computer next to the radio if you wait long enough you can see Cortana's face on the computer screen.