I am getting occasional marks appearing on 120 Tri-x negatives taken with a Rolleiflex. The marks are faint bars appearing at the bottom of images. The mark does not appear on all images; maybe one mark per film and does not necessarily appear on the same exposure number on the next film. When the problem does occur, the marks always appear at the bottom of the image. This is not a scanning problem as the mark appears on the negatives. Does anyone know what is causing these marks?