ATS Differential issues
 

So I have a 2014 Cadillac Ats with a power train Warranty and I heard a grinding noise so I took it to my mechanic. He adv that the Differential was shot and had metal in the fluid, So I take it to the dealership and not they say it's my wheel hub and that's not covered under warranty.They also said there is an "acceptable amount of metal shavings" in the fluid. I called my ASE certified mechanic back who swore it was the diff. and even listened to it with a stethoscope and said there was no noise from the wheels. So now I had to contact GM directly to get the diff replaced (hopefully). I am a loyal Cadillac customer (3) and never had warranty work done before. Is this commonplace when dealing with GM dealers? this may be my last Cadillac after this experience even though I love the product. These cars are not cheap and would expect excellent service as well.