That's the sound of a thousand souls whining cause they have to make room for more spartan and elite souls. All are with out heads so its more of a gurgling noise.
Or it is sarge greeting more newcomers to Hammerage in halo reach. Flips em around and stabs them with his combat knife. The only one who truly appreciated that welcome was Griff cause he was the only one who got back up. when he got back up, sarge started to cry cause he thought he was finally rid of Griff. No, Griff was fine and started to reexplore the canyon with simons.