Shutter capping. It needs a CLA. The second curtain catches up with the first curtain, closing the gap/slit before the travel across the film gate is complete.
“Believe nothing, no matter where you read it or who has said it, not even if I have said it, unless it agrees with your own reason and your own common sense.” – quote
I myself am made entirely of faults, stitched together with good intentions. -quote