My guess would be rust, too, or some other gunk on the inner side of the reels that hold the film, influencing the negative that is in immediate contact with the area. If you always spool your film more or less the same amount on your reel, spread out the film that has this problem and see if it's occuring in roughly the same place along the film. If so, spread out a roll of film (developed or undeveloped/bad film), mark the same location with pink marker, then spool it in the reel all the way to your usual amount. Find the place you marked, and with a magnifying glass, check the underside of the metal there for corrosion, etc. Take a toothbrush and give that spot a good brushing.