Hey Superfragl

Im a former 2008 Cadillac CTS owner and this was a common problem that was discussed at great length on our forum.

What we share with the CTS are these very wide rims. I personally chased this issue on my own car, it drove me nuts to say the least. If you have already proven out front end parts wheels and tires, there is one more thing to try that may help, Road Force Tire Balancing. I also invested in Michelin tires and could not have been happier with this brand. The vibration was reduced significantly but was never eliminated. The very weird thing about this was sometimes on the way to work (commuter) it disappeared and on another day I could feel it....pretty crazy stuff.

Best of Luck,

Joe