AC Voltage on Coaxial Cable (Cable TV)
If your cables are running parallel to a power cable they will induce a small voltage on the line. The cable should have a ground wire at the entrance of the house this is usually bonded to a metal water pipe.
Verify that the cable ground is going to a well grounded place (metal Pipe or Conduit that is grounded). If you do not see a ground wire from the external cable box at the entrance to the house it was not installed properly, a ground needs to be added and that should eliminate your problem.