wah lao....dangerous piece of info lurking around...no camera should have a sound when shaked unless u have a loose plastic lead for your camera memory card which is the only piece of equipment in a digital body that is not engineered for precision. Your buttons are all engineered to perfection and will not rotate like our old manual films...
It just don't make sense for a camera to have such a sound because nature photography requires u to have absolute silence unless u r taking photo....if u remember a show call jurrasic park, in one of the scene, a Nikon F5 user snapped at a dino and apparently it alarmed the beast launching attack.
Just ask yourself if u r using a 400D in the wild and u are walking or travelling with the camera on hand or shoulder and it rattles? U will only be shooting plants and no animals at all.
Get it repaired pal...before your warranty goes off....pose this question to your repairman if he insist this is normal.
PS : I dun own a 400D but I have a 5, 1v, 20D,30D and none of them rattles. If this model rattles then its time to go Nikon.