In NCAA football, what is the significance of the bong sounding on third down?

I'm not sure exactly what the significance is, or even whether it's universal from school to school.

Where I am (Rutgers University), the bell is sounded only when Rutgers is on defense, and it's really just a tradition to remind the fans to be as loud as possible while the opposing side is trying to plan and run their play.

Here is an example of the Rutgers third down "bong": https://www.youtube.com/watch?v=s6fxiT6zUsQ

[Edited to add link on September 19, 2016]

This is a coomon practice at USC:

The "Third Down Gong" at USC is supposed to start everyone cheering again. In my opinion, this isn't the best tradition to motivate the crowd.

More info can be found at: https://sports.stackexchange.com/questions/14855/origin-of-the-3rd-down-noise

new link added

I think the point isn't the specific bong, bell, sound or song used, it is the practice of trying to get the crown to cheer and be more engaged in the emotion of the game at a third down, especially if "your team" is on defense. The links others have given give examples of specific practices at Rutgers (bong) and USC (bells and songs). Many other schools have particular songs or cheers that are used for a defensive third down:

https://www.reddit.com/r/CFB/comments/2z0md9/what_are_your_schools_third_down_songs/

http://forum.siouxsports.com/topic/23322-new-third-down-songs-ideas/