I have a 2016 GMC Sierra All Terrain X. I have the same vibration issue that started almost immediately after purchasing the vehicle. It's been in the dealership four times and is no better than when it first started. The reason they are shimming the motor mounts is the change the frequency of the vibration so you don't feel it as much. They hook the vibration meter to a bolt that holds the seat down and shim away until the vibration lessons. Problem with this is it comes back because the root cause was not corrected. Most mechanics nowadays are only capable of remove and replace. If the computer does not tell them what's wrong they have no clue how to troubleshoot. Still having a problem and I'm getting ready for a fifth visit to the dealer. The way I see it is that there is the possibility of three different things that could be causing this. An air leak, a vacuum leak or an electrical short. If it's an air leak or a vacuum leak it's not very big. At idle or slightly above idle the leak is a larger percentage of the total air and fuel coming into the system. When RPM's are elevated the leak is a smaller portion of the air and fuel coming into the system and is virtually undetected. I also think it might be a problem with the way the vacuum system is setup. The system measures vacuum inside the manifold as well as out the manifold. If one system is using gauge measurements and the other is using absolute measurements it could cause problems at low RPM's. A gauge measurement is a linear equation and an absolute measurement is a quadratic equation. The bottom line with this is that the small differences in the method of measuring would cause problems at low RPM's but would be disguised at high RPM's due to the increased volume inside the manifold. The third issue of it being electrical would almost certainly cause the check engine light to come on so I have not gone down that road yet. Anyone else have luck getting their rough idle fixed?
