This thing is a rangefinder isn't it?
Rules out hole in the curtain I guess.
Is the streak on the top or bottom of the neg? IE:is it in the sky or foreground in horizontal shots?
Is it on the base or emulsion side?
If it's a scratch from the transport you will be able to see it by holding the neg at an angle to a desk or reading lamp.
If no scratches are apparent it's time to look for a light leak,. Does the shutter close completely? Can you shine a light into the camera both from front and rear and see a pinpoint of light through the shutter?
Do you experience blobs or dots of light if the film is left in one place for a longer period of time?