Caspien, my best guess is that its rubbing on the metal brace where the straps slide through. I wasn't sure what these straps did, but I'm pretty sure they keep the cover down at high speeds. I'm thinking maybe the latch catchers by the tailgate may have moved towards the cab by a hair over...
Not meaning to bring up a year old topic, but these particular straps on my Gladiator soft tonneau cover have both came apart where they go through the second brace from the cab. Looks like a dog chewed through both of them, but I'm sure they rubbed the bracing over the last four years and...