Help, 1997 XK8 with 41k miles. New sound and vibration on highway similar to when you drift out of your lane and the pavement has slott's cut into the road surface to make a noise and vibration to alert you, that's what's going on but not all the time. It comes and goes. Any suggestions?

 

That's not an insignificant vibration!

You say it comes and goes. Is that a regular harmonic vibration or a very random one?

 

Very rhythmic, repetitive and constant with no variation until it goes away

 

Is it a fast paced vibration? By this I mean does it seem to be at a higher rate/frequency than the rotation of the wheels for a given speed?

 

Does the frequency change with speed? If so, I'd start with tyres or a lost wheel balancing weight.

 

Yes very fast paced vibration. Feels like a constant vibration from transmission while driving and then suddenly goes away.

 

Tires and balancing are good, no vibration thru the steering wheel. Feel the vibration in the floor and throughout the car.

 

A fast paced harmonic vibration would lead me to look at the driveshaft couplers.

 

Does it by any chance only happen when you have started to drive from leaving the car say overnight?

 

Might be your transmission mount