During the daytime and only from Monday to Friday our house growls. The noise does not coincide with water being on or off, nor does it coincide with the ventilation kicking on or off. Sometimes the noise is low-pitched, sometimes high pitched. We have traced the noise back to the water pipe entry point in our crawlspace. When you hold the pipe you can feel the vibrations of the growl. I will stress this only happens during the day from Monday to Friday. Nights and weekends are growl-free. This is not your typical water hammer. On rare week days the noise is absent. Describing this to a professional plumber got me only strange stares and questions about animals in the walls. There is no animal living in our house that would make this sort of noise. Any takers?