For most teams, they want the most impressive aspect of their stadium's crowd interaction to be the loudness fans create. Whether it be the Seahawks or the Chiefs or whoever else, teams want their stadiums as loud as possible. For the Colts, however, their fans are perhaps best known for their silence.
When the Colts are on offense, the Colts fans get silent. This started back in the Peyton Manning era, when he would constantly be calling out audibles and signals at the line of scrimmage. He needed it quiet in order to communicate to his teammates most effectively, and Colts fans happily obliged as Manning almost always seemed to have the Colts in the right play. Of course, Manning helped fans to realize that he needed them quiet by motioning with his arms to quiet down, but the fans soon learned. And the RCA Dome would get about as quiet as any stadium in the league when Manning was at work. It was amazing to watch him work, and it was amazing to hear the fans quiet down so much to let him do his thing.
Lucas Oil Stadium doesn't get quite as quiet, but it's still pretty noticeable. Andrew Luck doesn't do as much as the line as Peyton did, but Colts fans are just used to it now. They're quiet when the team is on offense, and it's still something awesome to
Now, that's not to say that Colts fans don't get loud when they need to. That stadium can get electric at times, and ironically one of the loudest instances in Lucas Oil Stadium was last year when Peyton Manning returned, this time as a member of the Broncos. The fans roared to welcome Peyton back pre-game, and then when Peyton took the field as the opposing quarterback, the fans made plenty of noise as well. While it was still Peyton Manning, this time he was greeted with a loud fan base, and it was Andrew Luck who got the silent treatment (in a good way).
You'll find plenty of teams bragging about how loud their stadiums get. And Lucas Oil Stadium certainly gets noisy at times too. But one of the unique aspects of the Colts fan base is how quiet they get when the team is on offense.