Nikon D70: Noise on long exp: bright top left corner

[aculharobed]

The attached image was taken at night, with the camera under thick blankets in a dark room. The lens cap was on and the eyepiece was covered. Exposure was 2 minutes at f2.8, ISO 1600. With lower ISO speeds, it just takes longer to appear. At ISO 200, it takes 4 minutes to be visible, at 8 minutes it's quite clear. In-camera noise reduction gets rid of it. I also tried it with no lens, to eliminate it as a source of the problem, with the same results.

So the question is (since the guy at the camera store was useless):

Is it just my camera, or is this a "feature" inherent to all D70s?