It would be probably from the fences as you said that errors in winter way lower than in summer, unless there is something else used during summer in the house.. but you can use an AM radio on 612khz and around follow the picked up noise wave (if any) to the source, unplug it if noise gone then there it is (or one of the sources is gone).
A question that just pumped into my head, do your house has an electric fence? that is sharing power with the house, or where do the fences belong to?