I finally took the car in for service, I don't know why I waited so long! Here is what they found:

There was in fact a service bulliten 370107 on this issue. It seems that the Spring Accumulators were faulty. They replaced the 'sprung accumulators' and all is well. The car is tight over the bumps and no more strange noises!

Lesson learned, if it does not seem right, have it checked...