Ceramic floor tiles making a popping and cracking sound.

The most likely explanation is that the tile adhesive is no longer sticking to the concrete below and / or there are hollow areas beneath the tiles where there is insufficient tile adhesive. Either the tiler used the wrong type of adhesive for concrete floors or he did not ensure the concrete floor was dust free and cohesive before spreading the adhesive. Moreover he probably economised on the thickness of the bed of tile adhesive and / or did not ensure the original concrete floor was entirely level so that the problem tiles rest on less of a bed than the surrounding tiles. The first thing to do would be to remove the problem tiles and identify the cause before rectifying just this area or opting for a complete retiling job.