2001 Mercury Sable thumping bumping underneath car

For the past couple of years I have been trying to find out what the noise is underneath my car. I can hear it plainly when I slowly drive over small bumps, for instance, in a parking lot where there are small speed bumps or little potholes. I seems to happen when I turn my wheels to the right. Three different mechanics have tried to find the problem. I have had axles, tie rods, ball joints and I can't remember if there is something else replaced! Please give me some ideas for this problem. It is more like a rattle noise. Thanks, Charlene Fox