One very common problem with installing Saltillo tiles is that the backs of the tiles are not "back buttered" during the install. Saltillo tiles are usually a bit concave and with out this extra step will have a hollow space that sounds hollow when stepped on or tapped with something. The only way to fix it is to pull the tile and re-install with proper backing.

http://www.flickr.com/photos/kmswoodworks/44...