Hi Guys,
My wife's GEN III, 2001 XLS, has developed this vibration as if you are driving on pavers BUT the crazy thing is that it only does it in 4th gear. You can be driving normaly no vibration at low speeds and all of a sudden bbbrrrrrr like driving on pavers, now when it does that take your foot off the gas pedal and it goes away OR downshift to 3rd and it will ot vibrate.

I am thinking the culprit might be an engine support or transmission support. So I just checked the transmission support and it is fine so it coult be one of the engine mounts. The one on the passenger side is easy to check but the driver is not.

Have any of you had this issue? what was it?the mount?

Thanks,
Javy

All these new fangled car. How about u-joints, rear axle suspension bushings? In fourth gear, the harmonics will be caused by torque, low on the curve.

I'll take a wag and say you're lockup torque converter is malfunctioning.

Should I do a transmission flush?

Javy