[All-American ZL1]

I was really hoping that I didn't have to write this, but the situation has forced me to. I grew up in a Chevy family and have personally been a long time Chevy owner myself. I'm hoping that some of you could give me some advice on how to handle this situation.

I have a 2013 M6 ZL1. Like many other people I noticed the loud rear end. I had my dealership evaluate it around 5,000 miles, and they said it was normal. Over the next year I made some small modifications to the car, Pulley, headers, tune. I also used the car for exactly what it was advertised to do; I've done about 7-8 road track days, and I've had it down the strip maybe 30 times. This car is my prize possession, I do not beat on it, I am meticulous about taking care of it, inspecting it and maintaining it.

About 2 months ago when I hit 20k miles I had the dealership swap out the brake fluid and rear diff fluids for regular general maintenance. Last week the rear end started making a lot more noise so I took it back to the dealership. They contacted GM about the rear end noise, and they were told that GM would not warranty anything on the car since it has been modified.

I highly doubt that a pulley and headers would cause damage to a ZL1 rear end. We have numerous people on this site running 1,000 horses through their stock rear ends with no problems at all. Should I try a different dealership? I live in Tampa, FL. Thanks of your help.