It looks like light fall off from an obstruction, either in front of the lens or behind it. It's similar to, but too large to be, vignetting caused by diffraction of light along the edge of the film gate.

• After seeing the frame edge, it doesn't appear to be caused by the film gate because the ocean picture shows the border of the film gate within the shadow.

  When caused by the film gate, it's usually pretty small and cropped off from printing or mounting, so you don't have to worry about it because it's not visible. If the pressure plate is loose or film gate deformed, it can be larger.

• Some lenses have a frame behind the rear element. If the frame is not aligned with the film gate, it could cast a shadow on the film.

• I doubt a problem with the aperture itself because that would affect the image globally, not along an edge. A problem, like a loose blade, would be visible in bokeh balls.

• A shutter problem would have a sharper edge because it's pretty close to the film plane. They're also usually more obvious and variable. But this could be an early sign of impending shutter failure.

• Could be poor mirror clearance if the shadow always occurs along the same edge. However, the mirror isn't supposed to move until after shutter is closed.

• If the shadow occurs only when zoomed out, it could be related to imaging circle coverage, but I would expect the shape to be curved, like the imaging circle.