How To Cure a "Rappy" Exaust?

As others have suggested a crossover "H" pipe is the easiest fix.

You could also use "Stock" type muffler to choke the noise away,
but NAH, that would cut down on power.

I have No crossover either. :)

Hmm my youtube video isn't working for me, hope it works later.

http://www.youtube.com/watch?v=Ud8mci4rjFU