The wording of the question implies that this is 35mm.
The fact that the line appears in the same place in every frame, and that it doesn't extend all the way across the image, suggests that it is NOT a scratch in the emulsion. I suspect that it is either a hair or a bit of bristle from a lens brush that is lodged in place in the light path in the camera.
Are you sure the line is on the negative and not an artifact from scanning? Who did the scan, you or the lab?
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