What could be causing a very small voltage between mains grounding and a water pipe
The house ground is bonded to the supply ground at some point.  There might also be a ground spike and a connection to copper piping somewhere.

Although the ground circuit is not suppose to carry any current, it does due to capacitive or varistor leakage.  This current will cause a voltage drop and is what you measure.

Ask an electrician to ensure that the piping is bonded to the earth spike to be safe, if the regulations allow it.