The image doesn't show up for me. However, what you are describing sounds like "amp noise". Basically a portion of the sensor is getting some kind of interference due to the longer exposure times. My camera supposedly has it, although I have not made any exposures long enough to cause it yet. I think it uses the same sensor as my camera does though. One way to combat it is to turn your camera's Long Exposure Noise Reduction on. I don't know Sony camera's at all so refer to your manual on how to do that. It should remove the amp noise from the image as it writes the data to the card.