http://www.nikonchat.com/nikon-viewfinder-grid/
Scratch on my digital SLR sensor?
When I turn on my Nikon D70 and looking into the viewfinder, there is a line from the bottom center and ends in the center of the viewfinder. When I turn it off, it disappears. I have no option grid, and wondered what it was.
You may have a scratch in the mirror. A scratch on your sensor not shown in the display, as the mirror flips up for a moment and when blacks are allowed to image light to strike the sensor. A scratch in either front or rear lens elements may cause some distortion, but do not show a sharp line as you describe. It can be annoying but if it does not appear in his photographs, may not really need repair. Your decision.