There are three common items I know of that cause this problem
1. Metalic water pipe between houses/buildings with a common transformer serving them and one house has a high resistance grounded service conductor.
2. A high resistance grounded service conductor, simular to # 1.
3. A very low ground electrode impedance.
First thing I would do is turn off all the breakers and determine if the current is from an outside sources (like comming via water pepe) or generated within. That would give you a good starting point.
