If you are referring to a flat screen CRT set then I would take what you hear as a prior warning of a potential major breakdown in the high voltage section of the TV, this usually associated with the time base circuitry. Should at any time a single white line appear across the screen switch off instantly or you are liable to cause permanent damage to the phosphor on the tubes screen, this resulting in you being liable to see a permanent dark line across the picture which would be there for the entire life of the set. And just to reiterate, the above only applies to a set with a tube and not an LCD type.