knocking sound undercar???

I had a similar (if not the same) thing happen. I heard a knock from beneath the car which would happen more and more frequently especially at lower speeds.

I went to a dealership who kept trying to repair it, but finally said it was a normal driving characteristics. I went to another dealership and they fixed it in less than four hours. It showed out to be a loose "trans cross member" which the mechanic retorqued and the noise was immediately gone.