I have a 2011 S that I put a Koni suspension kit on. After the install, I found a vibration, that is more like a stutter and it happens only when under acceleration.
The vibration sometimes goes away only to come back later. It is worse when first moving in the morning, almost like a nylon tire after sitting...

The vibration feels like an unbalanced tire and on high speeds the stutter flattens out to a steady buzz. The weird thing is, if I lightly apply the brake when under power out of second gear, I can easily produce the stutter.

The clutch is fine, the tires are balanced and properly rotated. I moved the tires around to see if it was a bent rim. Could it be a bent axle?

 

Do you feel the vibration in the steering wheel? Definitely look at the control arm bushings as those could be a cause of vibration. Also, check axles, tie rods, ball joints and other suspension components for looseness/wear.

 

Thank you for your reply. I have checked all the related font end components. All the bushings, ends and tie rods are tight. No frame bushings or bearings are gone ether. The only thing left seems to be an axle. But at $600 a piece, I want to be sure.

Strange thing is, I can control the stutter, even when engaging the turbo, by how I accelerate.

 

May not be your issue really, but check your motor mounts for wear too. I did find this thread here that sounds very similar to your issue. No resolution though. Another thread here. Seems like all similar with suggestions as in regards to an axle issue. When searching, I've found about 5+ threads all with a similar complaint. All with axle's as the main cause and possible mounts, but replacing the mounts did not fully resolve the issue. Instead of buying new axles you could possibly find a drivetrain shop in your location that does rebuilds with OEM parts. I do this as I don't like dishing out the $400+ cost for new OEM axles and I wont go with cheap aftermarket axles as they tend to cause more issues.

 

This is great! Thanks for the threads. It seems like it could be an axle issue. Weird that this morning it was barely there...

 

My pleasure. It does seem like one of those weird issues. Maybe keep an eye on it and if it persists look to possibly getting them rebuilt. Let me know what you end up doing and what you find out. Good luck!