I have a 2004 XJ8 with less than 30,000 miles. I have rotated and balanced all the wheels recently and had local Jag mechanic look at this issue, but have not found a solution. At 60 mph or higher speed there is a noticeable low frequency beat and vibration. When I shift to neutral, the same vibration is there so it does not seem to be drivetrain related but either wheel bearing or perhaps brakes related. Longer I drive on the highway less the amplitude of this vibration. I did have airsuspension fault message come on when it was very cold this Winter but even when this went away, problem is still there. Would a rusted rotor and out of plane rotor cause this problem? Anyone with this experience?

 

Thanks everyone for responding and offering your suggestions and experiences. As an automotive engineer, I researched the past links as JagtechOhio suggested and also read a lot of the topics of orad induced vehicle vibration. Since the symptom does not go away even when I shift to neutral at 70mph going slightly downhill with no load on engine or the drivetrain, I had already concluded that it was wheel and or suspension related. Many have posted solutions or non-solutions to this problem from the past and they vary quite a bit, so there may not be just one root cause. So after much research I went to the local Firestone shop to have the Hunter GSP9700 road force wheel balancing done. You can read more on what this does on this link for those technicallly inclined - http://www.gsp9700.com/pub/technical/5098T/5098T.pdf. Bottom line is this balancing which takes about an hour for 4 wheels will catch issues that simple dynamic wheel balancing can not. My tires were out of balance by quite a bit and one tire had excessive road force measurement - all this after only one week of regular dynamic wheel balancing at another shop. Also to keep in mind is to make sure that your valve stems are properly lines up with the yellow marking on your tire - this match mounting is decribed here - http://www.tirerack.com/tires/tirete....jsp?techid=17.

After all this, the vibration has all but disappeared. My conclusion is that the combination of aluminum chassis and air suspension appear to create high sensitivity to wheel imblance (perhaps also to suspension bushing wear, wheel bearing wear etc.) as well as road surface noises, such that at higher speed, harmonic vibration of some order seems to set in. This techinique of doing everything you can possibly do to match balance all the wheels do appear to minimize this propensity for the post 2004 model year aluminum chassis Jag Xj8. Please try this solution and let me know if that also helps your problem. Thanks again for all the help. -Do Sohn