I recently purchased a 2006 XJ VDP with around 20,000 miles. At a constant speed of around 50 to 60 mph and only on very smooth roads I feel a slight vibration in the seat of the car along with a very faint hum, no shake in the steering wheel. With the tires fairly worn and winter being here I decided to put on new tires thinking that may be the problem. The slight vibration and faint hum is still there. The car acts fine under acceleration and deceleration and the symptoms are not noticable on normal or rougher roads or at lower speeds.
The car has lots of warranty but before taking it in I don't mind knowing what may be causing this. Has anybody have any suggestions as to what may be causing this?
Thank You,
brdave

 

I'd get the alignment checked. I bought my XJ used and had a similar issue, and an alignment did the trick (I also bought four new tires and it still vibrated). I had this issue with a prior car and the shop determined that I had slightly bent one of the rims (pothole perhaps). They couldn't get the rim to balance properly. The fix was to buy a new rim. Good luck.

 

A really good wheel/allignment shop can spot a bent wheel. You want a road force bal. on those tires too. If your tire shop doesn't do road force bal, find another that does. The price is about the same and given how sensitive these cars are to wheel/tire imperfections it's the only way to go. There are services that fix bent mag wheels on site for $125 to $150. I had a bent wheel fixed in a K Mart parking lot that saved me a pile of cash and heart burn.
Even if it turns out that wheel/tire issues aren't the problem you must start out with a base line to really get to the bottom of the vibration source.
Make a note of the weights and locations on each wheel. A thrown weight can really get you going in circles.