Is the problem due to improper installation?

I recently bought a new house. A travertine floor was installed in the master bathroom. The floor is now one year old.

On occasion the travertine tiles will creek when I walk across the floor. From day-to-day, the severity of this problem changes. On some days, the tiles will creek on almost every other step. On other days, the tiles will seldom creek. I've observed some cracking of the grout.

This floor is still under warrantee and I have spoken to my contractor. He is down playing the issue.

My questions are as follows:

Do I have a problem?

If I have a problem, is the problem due to improper installation?

If I have a problem, what are the potential causes?

Thanks,
Dave Mendrek