Buzzing from rear of car after new RDU installed – dealer doesn’t know why

A month ago I had a brand new RDU installed after I was having a grinding when turning sharp. After that repair, I started hearing a loud buzzing noise from the rear of my car anytime I’m above 35mph. It’s a constant noise regardless of acceleration, turning, etc. I took it back and they claimed it was a bent heat shield, but it still made the noise. Took it back a second time and they continue to claim it’s not an RDU issue even though that’s when I started hearing the noise. I found on…