Well, I already posted this on the Bungie.net forums, but they mostly gave me strange "you played Marathon?" looks, so no one could really confirm it for me.
Between 35 and 36 second into the trailer, immediately after the word "demons" (which is the word most clearly said by Gravemind), from the left audio channel, there is a quiet sound.
As far as I can tell, it appears to be the Marathon terminal login sound.
Another egg? Or just shadows on the cubicle wall?
Can anyone lend tru7h to these ramblings?