Car has a 489 case 8 3/4 rear with 3.55's on a cone style sure grip. Sometimes when backing up, the car will make a dull clunk and it comes from the rear. It makes this noise under power, but also has made it with the driveshaft off and being pushed. Where should I be checking? Axle bearings, center section or ?? Is this something the cone style sure grip does? It has been a long time since I have used anything other than the clutch style sure grip. Any advice is appreciated.

Thanks, Reid.