I dont know what this is but it sounds cool, like you would hear it from a big room which is far away:

http://www.youtube.com/watch?v=_OsD7Mz_oCE

2:38 start - slowly coming in
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I'd say it's simply a reverbed sound without the direct signal.
You can do this if you use an Aux send pre-fader to a Reverb and pull the actual track fader fully down (set the reverb on the aux track to 100% wet) or quicker in this case: use the reverb as insert and set it 100% wet.

Some things you can use in combination to pull a sound even more back:

http://kimlajoie.wordpress.com/2007/07/17/how-to-push-sounds -to-the-background/

far away reverb effect: more wet, less dry...

yes, pre-fade reverb, with the fader removed... other words, 100% verb with no direct signal... add some buck brigade delay before it and some compression after it... yay swarwing DubTech stabs

EDIT^ "pre-fade send reverb"

tnx!