Grinding/Scraping coming from the Rear
 

2005 CTS 3.6 liter, bought the car brand new. 2 weeks ago began hearing a grinding/scraping/rattling noise, sounds like a piece of metal being dragged across the pavement. The noise only occurs at around 1900 RPM, wondering if it has something to do with the transmission. It's been getting progressively louder throughout the past week. Any input would help, cars been in the shop already for 5 weeks due to an engine problem. Any input is appreciated.