Here's an article from NOLA.com that sums up the situation:
...this season at the Superdome, thanks to a rule change approved in March by the NFL competition committee, the Saints will have more leeway to encourage fans to let loose with as much noise as they can produce.
Under a rule that took effect in 1989, a team was not allowed to use video boards to show anything that would promote more crowd noise. Under the previous rule on club-controlled sound, "The use of noise meters or such messages as 'Noise!, ' 'Let's hear it!, ' 'Raise the Roof, ' 'Let's go Crazy, ' 'Pump it up, ' '12th Man' are prohibited at any time during the game. These examples are not limited to the foregoing, but also would include similar messages that encourage crowds to make random noise in order to disrupt the opposition."
...those restrictions no longer will apply until the play clock reaches 15 seconds.
Under the new rules, until the play clock hits that point, the Saints will be allowed to show any messages that aren't vulgar or derogatory on the video boards to encourage noise from the fans.
The Saints haven't offered specifics about what the team might do to get the fans' cheers in the Vikings' ears.
"We will certainly do everything within the rules to encourage fan participation and enjoyment of the game, " Saints vice president of communications Greg Bensel said in an e-mail.
The Atlanta Falcons have revealed a few of their ideas on how to promote fan noise.
Before key plays, the Falcons plan to show a decibel meter to encourage fans to push the indicator as high as possible.
There are restrictions under the new rules. For example, a team can't pump in noise to disrupt the visitors.
Under the new rules, "Any noise that is under club control must cease when the play clock is running, and the visiting team is in possession of the ball, or after the ball is kicked by either team on a kickoff. As an example, if the play clock is not running, a drumbeat, accompanied by "Defense" or some other message on the video board, may be played, but the drumbeat and any other audio must stop when the play clock starts while the visiting team is on offense."
Also under the new rules, videos of fans, cheerleaders and home-team players must stop when the play clock hits 15 seconds. Public-address announcements must stop when the huddle breaks or when the visiting team is ready to snap the ball in a no-huddle offense.
Also prohibited are videos of the visiting team's huddle, conferences in the visitors' bench area and the quarterback at the line of scrimmage.
The new noise policy stems from the NFL's effort to make the game more exciting for fans at the stadiums.
Commissioner Roger Goodell spoke at the league's annual meeting in March about how some fans might find it more comfortable to stay home and watch a game on television.
And here are the old rules, which I found on the Seattle Times web site from 2006. Even the wave was prohibited! (I blogged about the issue back about this time in 2006 as well.)