light tremble in head restraints while driving 60 to 90 miles

[stefspain]

I bought this convertible 2 years ago and then the car was pulling to the right. Wheels were switched front to rear and re aligned. Car drives straight out and steering wheel does not vibrate. However 5000 miles further, car trembles slightly on the motorway.
You feel it especially in the head restraint !
What can it be ?? Engine is running smoothly. No strange suspension noises at all.

[nordwulf]

I experienced a slight off-balance when tires were rotated or when tires are worn to a pint where they should be replaced. Perhaps having them balanced would fix the problem. You could also check the wheel bolts to see if they are correctly tightened. What kind of tires are they?