Most probably you have a pressure reducing valve malfunction. I have heard some various sounds from this problem through the years...from a sound of "popping corn", a train, and the moaning sound you described. The sound comes from vibration within the valve and echoes throughout the piping. You can use a pressure gauge made up to a hose fitting that will screw onto an outside hydrant (make sure the hydrant is installed after the PRV) or you can connect it to any sill **** or hose bib such as your auto washer connection. Make sure no one is running any water during your check. The pressure should read somewhere between 45-60 psi. If it is much higher or...if the pressure slowly rises, replace the PRV. You may have to leave the gauge hooked up for a period of time if the PRV is malfunctioning at a very slow rate or at various times. Another clue to this problem is when the water has not been used for a long time, such as overnight, and you turn it on in the morning, the pressure surges and then drops back to normal. Hope this helps.
