Sparky, is correct. There will always be current on the metal under ground water piping system in a code compliant installation if the water piping system is common to other buildings connected to the same utility transformer. In some cases, this path via the water piping system may even be a better path than the service neutral. Also remember that the current you are measuring on the water pipe may very well originate in a different building. This current exists even when there are no problems with the service grounded conductor. It will, of course, increase if there is a problem with the service neutral.
