1999 BMW 328 Sound

Suspension problem
1999 BMW 328 Engine Size unknown Wheel Drive Type unknown Automatic 120000 miles

I hit a large rock on the street and soon after I got on the freeway my car start making a grinding noise (hollow sound) which is slighlty felt while driving. I got off the freeway and checked under the car and saw nothing. As I continued to drive the sound decreased until I got to work and the sound was gone. I had a mechanic check the car out and we test drove it but there was no sound and he also checked underneath and saw no damage. He also checked the bearings and they were ok. Whenever I drive early in the morning the sound begins or if I hit a slight bump then I hear the noise, once my car is warmed up or I been driving for a while the sound decreases to a stop. If I get on the freeway and drive 60+ then the noise is more evident but it is not a permanent noise, it is continous though. It is also noticeable after I have come to a complete stop and start moving again. I am thinking it's either the shocks, or suspension, or axle and once the car warms up the noise decreases? I do believe me hitting the rock and the noise starting right after is related.