According to John Fuchs of the Carpet Warehouse, a hollow sound indicates a problem with the flooring under the tile. The sound occurs when you place tiles on an uneven floor. The tiles sit flat, but a small gap or air pocket sits just beneath the tile. As you walk across the floor, you hear a thumping sound caused by the echo made from your footsteps. A hollow sound can indicate that the tiles lack adequate mortar. The mortar holds the tiles in place, but when you do not use the appropriate amount, you create small air pockets under the tiles. The air pockets create the echo, which makes the hollow sound.
Repairing the problem requires removing the original tiles from the floor. Depending on the strength of the mortar, a crowbar or putty knife may be sufficient. When crowbar or putty knife is placed under the edge of the tile, you pop each tile loose from its original location. An adhesive solvent is strong enough to remove ordinary tile glue. For mortar, you must sprinkle water on the floor and use a rotary blade, which scrapes off the mortar without damaging the floor. Check the floor with a level and install the tiles in straight lines, working in small areas. Keep checking the tiles with the level, ensuring that the tiles sit evenly.
While water does not create hollow tiles, it can damage the tiles. Unsealed tiles with porous surfaces, including ceramic tiles, are susceptible to damage. The tile absorbs the moisture, which might lead to cracks, chips or crumbling spots. Water can also cause mold and mildew on sealed tiles, especially if you have a standing water problem.
Clean mold and mildew with bleach or a standard tile cleaner. Remove damaged tiles and replace them with new tiles. If you have unsealed tiles in the room, use a tile sealant on top of each one and on the surrounding mortar or grout. The sealant forms a protective barrier over the tiles, which blocks out water and moisture.