the tiles were improperly installed without a backer board or an uncoupling and load distribution membrane, and without that backer board stability, movement in the wood subfloor system. is causing the tiles to break bond.
To fix it properly would require tearing out the tile and reinstalling it or installing new tile with a backer board or uncoupling, load distribution membrane.
you should fix this ASAP as the tiles could crach when walked on sicne there is movement.
Since you probably wont be tearing it out to do it right, you can remove the grout, then pry up the loose tiles making noises, scrape existing hard thinset off the tiles, adjacent tile, and off the floor, sweep or shop-vac clearn, then re-adhere them using a cement based, latex modified thinset mortar, using 1/4"x1/4"x1/4", 1/4"x3/8"x1.4", or 1/4"x1/2"x1/4" notched trowel depending on the depth they originalluy used. You will have to match the trowel with that depth. Then re-grout with a similar or same colored grout, sponge even with surrounding joints, allow to dry, then boff tiles with a cloth to remove any haze grout residue.
They make inexpensive grout removal tools of many varieties for not much cash , and the materials wont cost much.
It is important to remove as much grout as possible so it makes the tiles easier to pry up without breaking the tiles.
Be careful about marring the finish or chipping the edges on the adjacent tiles.
here is a link listing many different carioeties of grout removal tools you can research.
one more thing ... since this floor was improperly installed, this probably wont be the last time you do this repair, so save and store the tools and materials you buy, becasue you will be using them again (and again) if you dont get this floor replaced.
There are two ways to do any job. The right way and the wrong way. Do it right everytime.