Now, a group of fans is trying to restore the roar.
When the unbeaten Chiefs welcome old rival Oakland to Arrowhead Stadium on Sunday, the fans will be trying to break a Guinness world record for loudest crowd roar in an outdoor sports stadium. The record was set earlier this year by Seahawks fans during a home game against the 49ers.
"Our fan base last year was divided 100,000 different directions. It was the worst I'd seen," said Ty Rowton, one of six fans who are spearheading the record attempt.
"The organization made a commitment to fans to make wholesale changes from top to bottom to turn this around," Rowton said, "and we felt with that commitment, us fans needed to rally around this team and come back together. Our mission was to restore Arrowhead."
It helps that the Chiefs (5-0) are off to the second-best start in their long history, and that their most bitter rivals are coming to town. Rowton and his crew - an eclectic group of fans, everything from a retiree to a construction worker to a grain biologist - decided on the Raiders because it stokes the passions of the Kansas City fan base like no other opponent.
The Chiefs organization is supporting the effort, too.
While fans paid the $700 fee to expedite approval of the record attempt, a team spokesman confirmed that the franchise paid for an adjudicator from Guinness World Records to fly to Kansas City for the game. Rowton said the cost of that was $7,500.
"We got a late start compared to Seattle. They had a four-month jump on us," Rowton told The Associated Press. "But we've got support on all levels, even the players are behind this."
The record had been held by Turkish soccer club Galatasaray S.K since March 2011, but the Seahawks pushed it to 136.6 decibels for their game against the 49ers. To put that in perspective, standing 100 feet away from a jet engine equates to about 140 decibels, and is the point where even short-term exposure can cause permanent hearing damage.
"It's tough to imagine that, man," Chiefs safety Eric Berry said. "Just the games I've been a part of it, it's already hard to get our calls. I can be standing right next to (cornerback Brandon) Flowers trying to get a call to him and he can't hear me. I'm screaming at the top of my lungs. So imagining it getting even louder is unimaginable."
Lest you think this whole endeavor is just a gimmick, there are very specific rules that must be followed to set a record, said Jamie Panas of Guinness World Records.
The sound level must be recorded on a "Class 1 precision measuring noise level meter certified and calibrated" for the attempt, Panas said. "A sound engineer must sign off on whether or not the measuring device is functional for this."
The microphone must be positioned between 1.5 and 1.6 meters off the ground (about five feet), and the record is measured by the general roar, which can include external instruments used by spectators.
"No mechanical noise produced by the venue will be taken into consideration," Panas said.
The NFL, though, has a policy that prohibits bringing noise makers into the stadium. Rowton and his posse are encouraging fans to purchase programs and roll them up to create MacGyver-style sound amplification devices, or simply to cup their hands to create a megaphone effect.
The weather could also play a factor in the attempt - windy days tend to dissipate the sound, so the fans are hoping for a still afternoon. The forecast for Sunday includes temperatures in the 70s with a clear sky and light breeze.
The biggest advantage Rowton's crew has is Arrowhead Stadium itself.
The acoustics of the 41-year-old building are such that sound tends to reverberate and amplify within the enclosed bowl. CenturyLink Field in Seattle, despite partial roofs that help to trap the sound, also has an open end zone that allows some of it to escape.
The seating capacity of 76,416 at Arrowhead Stadium, the fifth-largest in the NFL, also means that there will be nearly 10,000 more Chiefs fans than Seahawks fans trying for the record.
"I hope we do break the record with the noise level," Chiefs linebacker Derrick Johnson said, "but if we don't, I know it's going to be loud. I can barely hear when I'm out there. So as long as our crowd keeps that up, we'll be just fine."