Just wanted to share my experience in case it helps anyone still on here,
2007 Pontiac Grand Prix GT (supercharged). My issue started as a knocking noise obviously coming from the lower end of the engine. Shortly after, it started shuddering bad at specific speeds in different gears--notably 35+ mph in gear 2 and 3, and overdrive was shudder city. Two mechanics, just from listening to it, were convinced it was a spun rod bearing (and it really did sound like one). After finding out the trans fluid was low, filling it back up caused the knocking to stop. Finally, after getting it on a lift and looking through my nonexistent inspection cover, the flex plate was cracked.
It took a mechanic who is familiar with last-gen GPs to realize one of the transmission mounts was gone leading to the flex plate being stressed before it finally just snapped. They also told me the trans itself was probably bad due to metallic fluid but ended up just putting mine back on after I found out the trans from an 04 GP SC that we had got from a junkyard doesn't have the right wire connector for my harness (it has a 3 pin compared to my 4 pin) and the mechanic noticed that even the torque converter even looked pretty different.
After everything my car ran great again. I think reverse gear is starting to slip which sucks but she's alive for now.
