This comes with a strong spoiler alert up front. Only watch this, or read further, if you have seen the conclusion to Halo 3: ODST, or don't mind seeing it revealed.
With that advisory out of the way, reader Scazza filmed this Easter Egg (discovery credited to Loose on the Gamefaqs forums.) Hear that clicking? That's Scazza flicking left on the RIGHT analog stick.
If you do it during the epilogue's cutscene, the camera pans left at the conclusion - showing Buck picking a grub off what appears to be the Lone Monkey Man, a glitch/Easter Egg from Halo 3.
Halo 3 ODST Epilogue Easter Egg [YouTube]