I had the same problem with both of rear doors, tried everything to lube the straps only to find nothing really works. I actually took the strap and the bracket assembly off of the door last fall and found there was no cure to the problem but replacing them. I put it back and learned to live with that grinding noise and cheapy feel.
the strap runs through a bracket mounted in the door, and the bracket has a pair of spring loaded rollers in it, riding on the top and bottom surfaces of the strap. springs push the roller against the strap so they can "catch" or check the groove at half length of the strap when door is half open.
What I found in my car was a pair of completely worn rollers that no longer roll, they scratch the strap instead and make that ugly feel.
our front doors seem to have different parts in the brackets and hold up much better.
