Axle Assembly problems of the 1999 Lincoln Navigator

Grinding noise underneath vehicle's front end while application of power in acceleration or during climb on a hill. Brought to dealership and test drove the vehicle with mechanic and he could not hear a sound in the vehicle. The next day I took the same mechanic on another test drive on a hill where the sound could be heard. He found that the noise was inconclusive to any manufacture recalls and could not identify there was any problems. The noise is still there and he states there is nothing to worry about. I feel there is something wrong and do not want my family or my self to be injured because of something unidentifiable.