After listening to your video again. It still sounds like a rubbing flywheel.
Turn the Loco over. There should be a Phillips screw recessed into the bottom of each truck. See if one is loose or missing.
D500's explanation of the worm gear pitch would keep it from rubbing in reverse. In essence, in reverse the motor will be pulling itself into the truck due to the rotation of the worm turning the drive gear. This keeps the motor flush in its mount preventing the rubbing.
If the motor mount screws are there and tight. It could be a loose wiring harness hanging down from the shell making intermittent contact with the flywheel.
You could also try rotating the Loco 180 degrees on the track and see if it gets better or worse. The motors turn with the trucks something could be shifting in and out of the turns causing the rubbing on the flywheel.