Hey, 16 years old and working on a 1967 Mustang. Has a 351 Cleveland 4 carb. Was driving around and began hearing a loud thumping sound from the rear driver side that speeds up and slows with the wheel rotations. Went back and checked to see if there was a rock or something lodged in a brake housing or close to the rear axle, and nothing. Thinking it could be a worn down ball bearing inside of the axle. Any thoughts or suggestions are appreciated.