Reagan......the first one was a loud clonking (the loud knock you mentioned) sound which turned out to be a faulty seat latch on the rear seat squab......which was swapped under warranty.....as well as that they changed two other pieces of trim panel at the same time but every time they seem to cure one another one appears.......but I have noticed that it is more prevelent when one side goes over a bump rather than both which could be down to torque flex.....

But the X5 is one of the most rigid there.......