Do not turn down the main valve, it has no affect on water pressure. It will affect water flow and volume. Pressure is only controlled through a pressure reducing valve. The first question to ask is, what type pipes are in your home? If its galvanized piping, the problem could be coming from both a high pressure problem and a calcium buildup inside the pipes. Water in homes should never exceed 80 psi or flow faster thant 10 feet per second. Any time it does, it will create a water banging or growling noise when it runs. Since water builds up turbulance when it flows faster than 10 feet per second, the noise will only occur when the water runs. Test the pressure with a pressure gauge at one of the outlets. If the pressure is above 80 psi, you will need to install a pressure reducing valve and reduce it to 65 psi. If there is a restriction in the piping from a transition from copper to galvanized and back again, this will also cause a grouling because the water is traveling faster on the new piping and slower in the old piping. In the same manner that you put a sprayer on the end of the hose and the water sprays a great distance; in the inside of the pipe, if there is a restriction acting like a venturi, you will get a rumbling noise. Part of this noise is the turbulance created and part is the vibration and harmonic resonance created throughout the piping system like a tuba.