It just started happening a week or two ago.  When any of the fixtures are being run more than say halfway open (hot or cold) there is a groaning noise in the pipes.  It almost sounds like a gas powered line trimmer idling in the wall.  I've narrowed it down to a corner of the house, but I think it might be in the wall.

After looking online some, it's my understanding that this could be a loose fixture or washer and when you run water somewhere it reduces overall system pressure on that point which lets the thing vibrate a little.  Or...something like that.

Is there any trick to finding the culprit?