Lots of cars will make some pretty noticeable and bad diff noises if you change the fluid and do not use the proper additive. It is a like a tube of toothpaste in size and consistency. Add that, boom, problem gone. I do not know if the E9X M3 diff requires the additive. Given that folks have had the tranny fluid goofed by dealers I would not be surprised if this additive is required and the dealer left it out.
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