Sorry for the late followup. I did not have to pay for the transmission flush as they determined it was a bad torque converter. SM shared that he has seen “a number” of these failures. Issue was one of serviceability. The entire body had to be lifted in order to provide enough clearance to replace the Torque converter. Took them a few weeks, but when I got it back, the extreme noise was gone, and the transmission was shifting normally and quietly. Unfortunately, it was then I noticed the subtle shudder that was first unexplained and which I assumed would be taken care of when the transmission was repaired. Through my own testing during a cross-country trip and online research, I determined it was the dreaded V4/V6 shift shudder which of course GM is ignoring.
