sometimes when I fill a heater for the first time the water lines (assuming they are flexes/corrugated) make a high-pitched tone. Usually subsides after the tank is full. Maybe the water lines? Also, I once had a problem with a water pressure regulator near the water heater. It had debris in it and made a noise when the water would run through the piping. It turned out when the main water to the house was shut off to service the heater, some debris from the shut-off valve was knocked loose and gummed up the regulator. hope this helps.