Here is my opinion: when they get the ball, the crowd gets loud, as they approach the line for 1st down the crowd goes bananas. Watch 1st down then get load, as they approach the line the crowd goes bananas. Repeat for third down and forth down as needed. It's my understanding that the purpose of crowd noise is to disrupt their offense and encourage our defense. We get loud as they are coming to the line, while they are in the huddle calling the play we are either not as loud or not loud at all, waiting for them to break the huddle( to those that don't know, that's when the QB calls the play along with the count), if we were so loud that they can't hear the call wouldn't that help our defense even more. Crowd noise can create false starts, poor communication of play and count, impossible to check off(QB change play based off defensive alignment), and can force them to burn a timeout( unless we call one and bail them out, my bad, I couldn't resist), it's to our advantage if they burn up all their timeouts early.
Go Vols!
Edit: The purpose of third down for what is to let anybody that don't know when they have the ball stand and get loud for the reasons listed above, it does NOT mean "waive your hands in the air, like you just don't care", this ain't Opryland!