It was done in post production as far as I'm aware- they (very) briefly mention in the Quadrilogy that Cameron spent a great deal of time getting the sound fx just right.
I think it was music producer James Horner who said that- he was dismayed that Cameron was devoting more time to the sound of the "pulse guns" than to the score of the film.
Kevin
[img]http://groups.msn.com/_Secure/0TgDnAloXyJBkYBZV7s*Kj9!1Bc8b0FJ!WEmX8BpHJk8yRyABH697OsLy1Bm1knQrHcDDdHqDnRAmyoDB6vy6w!ASmCM57J8W6cyCxY8Y84TWwwCw7UVNAA/Burton%20004.JPG[/img]"I know I'm human. And if you were all these things, then you'd just attack me right now, so some of you are still human. This thing doesn't want to show itself, it wants to hide inside an imitation. If it takes us over, then it has no more enemies, nobody left to kill it. And then it's won."