We've isolated the crowd audio on the 49ers' blocked punt in the first quarter. At the 19-second mark of the video, you can here a few short, sharp whistles in quick succession—exactly the cadence of a referee trying to blow the play dead.
Several Seahawks, especially on the left side of the line, stood up and looked around, thinking there was no play. It was on that side that Craig Dahl came around to block Jon Ryan's punt.
Seattle fans are attempting to break the Guinness World Record for crowd noise tonight. Maybe some are a little too excited about it.