Is this normal for my mustang to do this?

have a 2013 ford mustang v6 with about 38k miles. Automatic. When im driving lets say in a parking lot I let off the gas and the engine rpms kinda stay right where they were before I let off. They do get lower very slowly. But when I hit tye brake they lower alot obviously. Is this normal for the rpms to stay arounf where they were before letting off? Also if im coasting downhill at very low speeds like 20 mps I will hear the rpms surging up and down by about 200 rpm
Allso im hearing a rear end clunk when accelerating from a stop or roll. The ford tech said its differential backlash and its normal by why would it be clunking?

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