Differential noise - 74 Spider; 40k miles
I'm looking for a layman's explanation as to what is causing my rear end to make a loud growling noise. Within a week of purchasing the car I had it in the shop to get the differential gaskets changed. There was a large amount of grease caked on the unit. However, it was not making any noise. The shop checked the gears for play and made sure the nut was tight. They filled the differential with a synthetic gear oil. Then the noise started. I took it back and had the lubricant changed to an 80/90 GL 5 conventional gear oil. It was not Shell. The noise did not stop.
It occurs when you unload the drivetrain by cutting back on the gas or giving no gas and coasting. It does not happen during acceleration.
From a mechanical stand point what is going on inside the differential to cause the growling noise?