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You are right of course - a differential would make a noise cornering. The mechanic was using a bit of tradie slang. What he should have said was "final drive" to be more accurate.
What you have is some wear in the pinion bearings in the final drive. As the thrust changes from driving to overrun when the accelerator is released you get some noise. It's common after a while on all rear wheel drive cars.
It's going to mean a final drive rebuild and that's costly. You could add some aftermarket product like an oil stabilizer to ward that off for a while - some stp for final drives for instance - however sooner or later it will need a rebuild.