Added after a particularly poor, old or obscure joke, the BoomBoom tagline originated in Music Hall, where a bass drum was struck twice to indicate to an audience that the previous line was indeed a joke. Some people claim that laughter tracks on TV shows are intended to accomplish the same aim, whereas others point to research that shows that laughter tracks "work" because laughter is a primarily sociological phenomenon, and not simply because they indicate where the "jokes" were(n't).
[However, American laughter tracks are sometimes omitted when shows (eg., MASH) are reshown on British television.]
Q: what do you get when you throw two elephants off a cliff?
A: BoomBoom
see also RimShot?
see also Basil Brush: http://www.bbc.co.uk/cult/ilove/tv/basilbrush/intro.shtml