Why do pipes vibrate when you turn on the water?

It's simply the pressure of the water. Mine do the same thing. There is nothing wrong with the pipes. Once a month I use either a box of baking soda, or bathroom scum cleaner and let my washing machine go through a full cycle. This gets ride of any built-up soaps or fabric softeners. Often the vibration and 'water hammers' are due to air in the pipes. The other thing to check out is piping vibrating against the joists.