Drummy or hollow sounding tiles can easily be detected by tapping over the tile surface. Where the sound is hollow, this is due to the adhesive not having a good bond between the tiles and the substrate or there being "voids" of missing adhesive under the tiles. Tiles which sound drummy are no longer adhered properly and may dislodge. In the case of floors subjected to foot traffic, the tiles that are not correctly laid will most likely break, crack, chip or fail in some other way. In extreme circumstances, the tiles will actually lift off the floor. This is usually due to old adhesives, incorrect tiling or tiles laid too close together with no control joints to allow for expansion.

WIKIPEDIA ENTRY FOR TILE.