i have a similar problem in my house. the water in washer machine gives you a shock from time to time. i checked the outlet, panel, meter, and grounding system. nothing wrong there. i found out later that in 1992, my whole nieghborhood had been demolished by hurricane andrew. a lot of unlicensed contractors did a lot of work in the area when they rebuilt. upon remodeling the bathroom 2 yrs ago i found a neutral and grd together in the vanity light box floating in the wall. so there is evidence of wrong doing but i still havent found the problem. its a concern but not major one to remodel my whole house.