Origin of the 3rd Down "noise"

The theory is that crowd noise hurts the offense and helps the defense, which is most critical on 3rd down, where the offense is usually facing their last chance for a 1st down.

The offense has to coordinate several things in order to make a successful play. The play caller (usually the head coach or offensive coordinator) needs to select a play from the sidelines and communicate that play to the quarterback, usually via a headset radio. The quarterback then has to communicate that play to the rest of the offense, usually verbally in a huddle. Next, the quarterback may need to change the play at the line, which he communicates by yelling an audible call to the team. Finally, the quarterback's vocal cadence signals to the rest of the offense team when to start the play.

Loud crowd noise can disrupt any of these communications.

Teams like Seattle and Minnesota have a reputation for having very loud stadiums. Seattle believes in the advantage that their crowd gives them so much that they refer to the fans as the "12th Man" and have retired jersey number 12 in honor of the fans. Minnesota and other stadiums have been accused of pumping amplified crowd noise through stadium loudspeakers to gain an advantage.

In 1989, the NFL felt so strongly about the disadvantage that the visiting offense faced that they instituted a penalty for excessive crowd noise, issued against the home defense. The referees in enforcing the rule could take away timeouts and issue penalties, 5 yards at a time, until the crowd quieted down. The new rule also banned stadiums from encouraging noise via announcements. The rule was not very popular, and officials eventually stopped enforcing the rule. The penalty was officially dropped from the rulebook in 2007, and video scoreboard announcements encouraging noise were allowed again in 2013.

A sound effect played by the stadium before a third down is simply a cue to the crowd that the visiting team is in a third down situation and that they should be loud to help their team. Different stadiums have different traditions for these sound effects: Denver has a bronco neighing, Minnesota has a Viking horn, Jacksonville has a jaguar roaring. They usually play these after a good play by the home team, but they can also be used to alert the crowd to a third down situation.

The origins of these noises goes back to the beginnings of the games. You simply want as much noise as possible when the other team has the ball. In the playoffs this might be every play of the game.

The NFL has specific revisions written that allows a team to pump certain amount of noise. Numerous teams have broken the volume/timing rules and have had to pay fines. The generated noise helps fans get loud themselves, because they can't hear themselves - or how loud they are.

What effect does noise have?

• as mentioned in another answer, it is harder to call the play. On a scale of 1 to 10 this is probably a 2-3 though. Teams know things will be loud and they are ready for this. Also a lot of teams use hand signals and cards to call plays.
• the next thing is audibles. Again you can use hand signals but this probably has a much bigger effect. There is only a certain amount of things a QB can do with his hands, without giving the audible away. On a scale of 1 to 10, this is around 5-7.
• now the biggest deal and a solid 9 out of 10 is the snap count. If you can't hear the QB, you could jump offsides. 5 yards is a big deal and a jump can stall a drive. On the flip side the lineman might have to watch the ball and go on movement. This eliminates the one advantage an offense has. I have had several DL that have played for me that have a quick first step and would destroy on offensive lineman (usually bigger, slower) if they were on a level playing field. There are players in the NFL that have made a career of jumping the snap count 5-10 times a game.