banging pipes

Every time we shut off running water from any of our faucets, a load banging sound eminates from the pipes, sometimes as many as 10-20 bangs. What is actually happening to have it make such a sound? On several occasions I have shut off the water main and drained the air chambers. That helps a little, but then within days its back to the usual banging. Could it be that the chambers have not drained completely, or is there such a thing as a leak in the chamber that allows it to refill with water? I realize you dont know the layout of our house, but is there any way to fix that without tearing up walls? Should I look in the crawl space below the house for any clues?