Thinking about it a bit, that sounds like a tranny/torque converter issue. Try this... put the vehicle in park and rev the engine then let off the gas as you would on the highway; if you do not hear the sound, the issue is not engine related. It would be something drive-line related. Also, do you hear this sound under lower speed conditions?
My intuition says if it doesn't happen in park, and doesn't happen at say below 40 mph... it might be the torque converter having some problems moving out of lock-up.
Another test would be to put the vehicle in neutral when it makes this sound and see if it goes away. (but remember, it would be advisable to come to a stop before moving the shifter back to drive... unless you've got a manual LS... in which case, disregard all of this about torque converters!)
