It's the orientation sensor. You'll find the same thing with *many* camera models. Some of the Canon manuals even mention it. For example, page 105 of the XT manual says this:
"When you change the camera’s orientation between horizontal and vertical, the camera orientation sensor will make a small sound. This is normal and not a defect."
http://pdisp01.c-wss.com/gdl/WWUFORedirectTarget.do?id=MDkwMDAwMDI3ODAx&cmp=ABR &lang=EN',%20'EOSDRXT350DIM-EN.pdf
