Do you process your own film or send it out?
as stated could be pressure plate,
could be dust in the camera
if your film is being sent out the cause could be at the proofing stage, poor handling .
if your film is being sent out , and the lab uses a roller transport film machine for the black and white this also could be the source.
If you are processing yourself and are not photoflowing correctly I have seen a white line in the print that runs the lenght of the film in the last few frames.
If you send to a lab , check what type of machine .
If you process yourself then I suspect the camera.
best regards Bob