Cadillac STS Forum - 2005 through 2012 Discussion, Acceleration Noise in Past Cadillac Vehicle Discussion; Originally Posted by orange57
Check your motor mounts and your diff bushing. If you Google STS diff bushing replacement there ...
Depends on if you DIY or take it to the shop. The stock Diff bushing is garbage, and I think book time is like 10 hours to replace it. Creative Steel makes a heavy duty bushing and rents a tool that allows you to replace it with the diff still in the car.
Motor mounts are around $125 each, IIRC. Not sure what shop charges for time on these.