Hard to explain...

I have an HP Scanjet 5500c When I scan a picture, the image is scanned but so is the "gears or working parts" of the scanner itself. So I get an image of metal with holes in it - part of the inside of the scanner.

I just purchased one of these scanners secondhand, and I have the same problem. You're seeing the underside of the lid where the film strip conveyors and other mechanicals are located. The previous owner (original purchaser) had a piece of white printer paper taped to the underside of the lid in order to prevent this effect. I'm really baffled as to why HP didn't install an insert for users who don't scan photographic prints and film strips.