Water hammer is a banging or thumping in water lines. The noise occurs when the flow of moving water is instantaneously stopped by a closing valve. RonP, did changing the valve permantly fix the problem for you?

You might have a large object stuck in your drain pipe under your bathroom. This same type of problem happened to me recently. I talked to a plumber and he said to fill up all the sinks and bathtub in that bathroom, then flush your toilet and drain your sinks/bathtub at the exact same time. This creates the maximum amount of water pressure on the object - flushing it out of the way.