thanks for the suggestions - I will look into these possibilities. it feels to me like scratching is the likely cause. If something is blocking the light it would have to be a very thin hair or thread, and this is unlikely since looking back the problem has been occurring for at least the last 10-20 rolls. I just haven't noticed it because it is virtually hidden unless the negative is very dense along that side, and also I usually crop to 4x5 so I don't pay much attention to the very edges of the long sides of the neg.