There is vibration problem when I drive over small bumps in the road. It is NOT a mettalic sound but more of a thud, thud thus sound. I changed the struts but it the sound is still there. We are not able to determine where the vibration is coming from. The mechainc reviewed all the mounting from the exhaust onwards, He did some work on the undercarriage, all the possible vibrations were all checked and eliminated The tires are new and recently rotated. So that leaves the steering or the bearings (in my mind). The vibration is coming from road (Wheels to bearings to struts to steering to wheel mounting) to the steering. After $ 800.00 I am still no closer to the solution. I will really appreciate your advice and imput.

Gm has had issues with the steering colomn shafts and they get dry and cause this type of thud and vibration. Usually the mechanic just pumps the shaft full of grease to quite it down and that helps for a while. Have them check the sterring shaft for this condition as well as the ball joints. This condition is worse in trucks but it can be found in most GM cars as well over time.

If you have radials and you cross rotated you have a tire about to orange peal on you. Radials should only be rotated front to rear on the same side of the car they started on. Inspect the tires for a bubble or being over inflated. If that is good you need to check the ball joints and upper A-Frame bushings. all of this assumes the noise is in the front end. A rear shock will give you the same noise and symptom if it has lost it's charge so check those as well. Finally, if you have front wheel drive check the CV-Joints and boots.