Richie1976 is right
They did this experiment on Braniac on Sky One:
http://www.g4tv.com/brainiac/episodes/4578/Epi sode_15_Brown_Noise.html
Brown Noise, they explained, was a sound that resonated at the same natural level as your colon, thereby stimulating your poop. They even told you the frequency and decible level you need to play it.
They played it to a guy in a portaloo, then went back to see what had happened. Actually, they never really revealed what had happened, the presenter just made some comment like, "better get the mop".