I had a similar issue with the phone line when we moved into the house we are now. When it would rain, phone would be scratchy and sometimes not work at all. CenturyLink sent someone out and we found that the old owner had started to run a line somewhere else but never finished. He coiled up the cord in a plastic grocery bag and there it sat, covered up by leaves for who knows how long. And it would short out every time the rain would come.
Disconnected that old line, threw it in the trash, and that was that.